selftests/bpf: Merge most of test_btf into test_progs

Merge 183 tests from test_btf into test_progs framework to be exercised
regularly. All the test_btf tests that were moved are modeled as proper
sub-tests in test_progs framework for ease of debugging and reporting.

No functional or behavioral changes were intended, I tried to preserve
original behavior as much as possible. E.g., `test_progs -v` will activate
"always_log" flag to emit BTF validation log.

The only difference is in reducing the max_entries limit for pretty-printing
tests from (128 * 1024) to just 128 to reduce tests running time without
reducing the coverage.

Example test run:

  $ sudo ./test_progs -n 8
  ...
  #8 btf:OK
  Summary: 1/183 PASSED, 0 SKIPPED, 0 FAILED
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200916004819.3767489-1-andriin@fb.com

tools/testing/selftests/bpf/.gitignore

@@ -15,7 +15,6 @@ test_sock
 test_sock_addr
 test_sock_fields
 urandom_read
-test_btf
 test_sockmap
 test_lirc_mode2_user
 get_cgroup_id_user

tools/testing/selftests/bpf/Makefile

@@ -33,7 +33,7 @@ LDLIBS += -lcap -lelf -lz -lrt -lpthread
 # Order correspond to 'make run_tests' order
 TEST_GEN_PROGS = test_verifier test_tag test_maps test_lru_map test_lpm_map test_progs \
 	test_verifier_log test_dev_cgroup test_tcpbpf_user \
-	test_sock test_btf test_sockmap get_cgroup_id_user test_socket_cookie \
+	test_sock test_sockmap get_cgroup_id_user test_socket_cookie \
 	test_cgroup_storage \
 	test_netcnt test_tcpnotify_user test_sock_fields test_sysctl \
 	test_progs-no_alu32 \