During the second phase of select - after all cases were polled and
noone was found to be ready - cases are queued to corresponding channels
recv and send queues. While that queueing is in progress, a case, that
was already queued, could win (see f0b592b4 "select: Don't let both a
queued and a tried cases win at the same time" for details).

If such won case is detected, we break out of queuing loop, but
currently don't wait for that case to become ready. This is a bug,
because when a case is marked as won, its data is not yet copied - for
example for won recv case if we don't wait for that case to become
ready, we will be returning from select while corresponding *prx and
*rxok for recv waiter is still being copied in progress.

An example TSAN reports for this bug are as follows:

(1) WARNING: ThreadSanitizer: data race (pid=8223)
  Read of size 1 at 0x7b1800000a48 by main thread:
    #0 __chanselect2 golang/runtime/libgolang.cpp:1112 (liblibgolang.so.0.1+0x5fd6)
    #1 _chanselect2<true> golang/runtime/libgolang.cpp:949 (liblibgolang.so.0.1+0x6665)
    #2 _chanselect golang/runtime/libgolang.cpp:944 (liblibgolang.so.0.1+0x6665)
    #3 __pyx_f_6golang_7_golang__chanselect_pyexc golang/_golang.cpp:5896 (_golang.so+0x1deac)
    #4 __pyx_pf_6golang_7_golang_4pyselect golang/_golang.cpp:4935 (_golang.so+0x1deac)
    #5 __pyx_pw_6golang_7_golang_5pyselect golang/_golang.cpp:4355 (_golang.so+0x1deac)
    #6 PyEval_EvalFrameEx <null> (python2.7+0xf0e49)

  Previous write of size 1 at 0x7b1800000a48 by thread T57:
    #0 golang::_RecvSendWaiting::wakeup(bool) golang/runtime/libgolang.cpp:346 (liblibgolang.so.0.1+0x459d)
    #1 golang::_chan::_tryrecv(void*, bool*) golang/runtime/libgolang.cpp:730 (liblibgolang.so.0.1+0x511d)
    #2 __chanselect2 golang/runtime/libgolang.cpp:1074 (liblibgolang.so.0.1+0x5d4b)
    #3 _chanselect2<true> golang/runtime/libgolang.cpp:949 (liblibgolang.so.0.1+0x6665)
    #4 _chanselect golang/runtime/libgolang.cpp:944 (liblibgolang.so.0.1+0x6665)
    #5 __pyx_f_6golang_7_golang__chanselect_pyexc golang/_golang.cpp:5896 (_golang.so+0x1deac)
    #6 __pyx_pf_6golang_7_golang_4pyselect golang/_golang.cpp:4935 (_golang.so+0x1deac)
    #7 __pyx_pw_6golang_7_golang_5pyselect golang/_golang.cpp:4355 (_golang.so+0x1deac)
    #8 PyEval_EvalFrameEx <null> (python2.7+0xf0e49)
    #9 <null> <null> (python2.7+0x1929e3)

  Location is heap block of size 96 at 0x7b1800000a20 allocated by main thread:
    #0 calloc ../../../../src/libsanitizer/tsan/tsan_interceptors.cc:623 (libtsan.so.0+0x2b323)
    #1 calloc ../../../../src/libsanitizer/tsan/tsan_interceptors.cc:618 (libtsan.so.0+0x2b323)
    #2 __chanselect2 golang/runtime/libgolang.cpp:1018 (liblibgolang.so.0.1+0x5b8c)
    #3 _chanselect2<true> golang/runtime/libgolang.cpp:949 (liblibgolang.so.0.1+0x6665)
    #4 _chanselect golang/runtime/libgolang.cpp:944 (liblibgolang.so.0.1+0x6665)
    #5 __pyx_f_6golang_7_golang__chanselect_pyexc golang/_golang.cpp:5896 (_golang.so+0x1deac)
    #6 __pyx_pf_6golang_7_golang_4pyselect golang/_golang.cpp:4935 (_golang.so+0x1deac)
    #7 __pyx_pw_6golang_7_golang_5pyselect golang/_golang.cpp:4355 (_golang.so+0x1deac)
    #8 PyEval_EvalFrameEx <null> (python2.7+0xf0e49)

  Thread T57 (tid=13758, finished) created by main thread at:
    #0 pthread_create ../../../../src/libsanitizer/tsan/tsan_interceptors.cc:915 (libtsan.so.0+0x2be1b)
    #1 PyThread_start_new_thread <null> (python2.7+0x19299f)
    #2 _taskgo golang/runtime/libgolang.cpp:123 (liblibgolang.so.0.1+0x3f98)
    #3 __pyx_f_6golang_7_golang__taskgo_pyexc golang/_golang.cpp:5926 (_golang.so+0x16f7e)
    #4 __pyx_pf_6golang_7_golang_2pygo golang/_golang.cpp:2399 (_golang.so+0x16f7e)
    #5 __pyx_pw_6golang_7_golang_3pygo golang/_golang.cpp:2324 (_golang.so+0x16f7e)
    #6 PyEval_EvalFrameEx <null> (python2.7+0xf0e49)

(2) WARNING: ThreadSanitizer: data race (pid=14185)
  Write of size 8 at 0x7b1800003000 by thread T95:
    #0 free ../../../../src/libsanitizer/tsan/tsan_interceptors.cc:649 (libtsan.so.0+0x2b46a)
    #1 free ../../../../src/libsanitizer/tsan/tsan_interceptors.cc:643 (libtsan.so.0+0x2b46a)
    #2 operator() golang/runtime/libgolang.cpp:1023 (liblibgolang.so.0.1+0x44bd)
    #3 _M_invoke /usr/include/c++/8/bits/std_function.h:297 (liblibgolang.so.0.1+0x44bd)
    #4 std::function<void ()>::operator()() const /usr/include/c++/8/bits/std_function.h:687 (liblibgolang.so.0.1+0x5fd8)
    #5 golang::_deferred::~_deferred() golang/runtime/libgolang.cpp:215 (liblibgolang.so.0.1+0x5fd8)
    #6 __chanselect2 golang/runtime/libgolang.cpp:1023 (liblibgolang.so.0.1+0x5fd8)
    #7 _chanselect2<true> golang/runtime/libgolang.cpp:949 (liblibgolang.so.0.1+0x6736)
    #8 _chanselect golang/runtime/libgolang.cpp:944 (liblibgolang.so.0.1+0x6736)
    #9 __pyx_f_6golang_7_golang__chanselect_pyexc golang/_golang.cpp:5896 (_golang.cpython-36m-x86_64-linux-gnu.so+0x1e562)
    #10 __pyx_pf_6golang_7_golang_4pyselect golang/_golang.cpp:4935 (_golang.cpython-36m-x86_64-linux-gnu.so+0x1e562)
    #11 __pyx_pw_6golang_7_golang_5pyselect golang/_golang.cpp:4355 (_golang.cpython-36m-x86_64-linux-gnu.so+0x1e562)
    #12 _PyCFunction_FastCallDict Objects/methodobject.c:231 (python3.6+0xd4db9)
    #13 pythread_wrapper Python/thread_pthread.h:205 (python3.6+0x6c5d6)

  Previous read of size 8 at 0x7b1800003000 by main thread:
    #0 golang::_RecvSendWaiting::wakeup(bool) golang/runtime/libgolang.cpp:347 (liblibgolang.so.0.1+0x4769)
    #1 golang::_chan::_trysend(void const*) golang/runtime/libgolang.cpp:661 (liblibgolang.so.0.1+0x5781)
    #2 _chanselect golang/runtime/libgolang.cpp:901 (liblibgolang.so.0.1+0x64d9)
    #3 __pyx_f_6golang_7_golang__chanselect_pyexc golang/_golang.cpp:5896 (_golang.cpython-36m-x86_64-linux-gnu.so+0x1e562)
    #4 __pyx_pf_6golang_7_golang_4pyselect golang/_golang.cpp:4935 (_golang.cpython-36m-x86_64-linux-gnu.so+0x1e562)
    #5 __pyx_pw_6golang_7_golang_5pyselect golang/_golang.cpp:4355 (_golang.cpython-36m-x86_64-linux-gnu.so+0x1e562)
    #6 _PyCFunction_FastCallDict Objects/methodobject.c:231 (python3.6+0xd4db9)

  Thread T95 (tid=16547, running) created by main thread at:
    #0 pthread_create ../../../../src/libsanitizer/tsan/tsan_interceptors.cc:915 (libtsan.so.0+0x2be1b)
    #1 PyThread_start_new_thread Python/thread_pthread.h:252 (python3.6+0x6c67e)
    #2 _taskgo golang/runtime/libgolang.cpp:123 (liblibgolang.so.0.1+0x4158)
    #3 __pyx_f_6golang_7_golang__taskgo_pyexc golang/_golang.cpp:5926 (_golang.cpython-36m-x86_64-linux-gnu.so+0x1a9b5)
    #4 __pyx_pf_6golang_7_golang_2pygo golang/_golang.cpp:2399 (_golang.cpython-36m-x86_64-linux-gnu.so+0x1a9b5)
    #5 __pyx_pw_6golang_7_golang_3pygo golang/_golang.cpp:2324 (_golang.cpython-36m-x86_64-linux-gnu.so+0x1a9b5)
    #6 _PyCFunction_FastCallDict Objects/methodobject.c:231 (python3.6+0xd4db9)

-> Fix it by always waiting for WaitGroup's won case to become ready.

The bug was introduced in 3b241983 (Port/move channels to C/C++/Pyx). Before
that - when channels were implemented at Python level, we were always waiting
on select's group.

Added test catches the bug on all - even not under TSAN - builds.