kunit: tool: suggest using decode_stacktrace.sh on kernel crash

kunit.py isn't very clear that
1) it stashes a copy of the unparsed output in $BUILD_DIR/test.log
2) it sets $BUILD_DIR=.kunit by default

So it's trickier than it should be for a user to come up with the right
command to do so.

Make kunit.py print out a command for this if
a) we saw a test case crash
b) we only ran one kernel (test.log only contains output from the last)

Example suggested command:
$ scripts/decode_stacktrace.sh .kunit/vmlinux .kunit < .kunit/test.log | tee .kunit/decoded.log | ./tools/testing/kunit/kunit.py parse

Without debug info a user might see something like
[14:11:25] Call Trace:
[14:11:25] ? kunit_binary_assert_format (:?)
[14:11:25] kunit_try_run_case (test.c:?)
[14:11:25] ? __kthread_parkme (kthread.c:?)
[14:11:25] kunit_generic_run_threadfn_adapter (try-catch.c:?)
[14:11:25] ? kunit_generic_run_threadfn_adapter (try-catch.c:?)
[14:11:25] kthread (kthread.c:?)
[14:11:25] new_thread_handler (:?)
[14:11:25] [CRASHED]

`tee` is in GNU coreutils, so it seems fine to add that into the
pipeline by default, that way users can inspect the otuput in more
detail.

Note: to turn on debug info, users would need to do something like
$ echo -e 'CONFIG_DEBUG_KERNEL=y\nCONFIG_DEBUG_INFO=y' >> .kunit/.kunitconfig
$ ./tools/testing/kunit/kunit.py config
$ ./tools/testing/kunit/kunit.py build
$ <then run decode_stacktrace.sh now vmlinux is updated>

This feels too clunky to include in the instructions.
With --kconfig_add [1], it would become a bit less painful.

[1] https://lore.kernel.org/linux-kselftest/20211106013058.2621799-2-dlatypov@google.com/Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/kunit/kunit.py

@@ -156,6 +156,12 @@ def exec_tests(linux: kunit_kernel.LinuxSourceTree, request: KunitExecRequest,
 
 		test_counts.add_subtest_counts(result.result.test.counts)
 
+	if len(filter_globs) == 1 and test_counts.crashed > 0:
+		bd = request.build_dir
+		print('The kernel seems to have crashed; you can decode the stack traces with:')
+		print('$ scripts/decode_stacktrace.sh {}/vmlinux {} < {} | tee {}/decoded.log | {} parse'.format(
+				bd, bd, kunit_kernel.get_outfile_path(bd), bd, sys.argv[0]))
+
 	kunit_status = _map_to_overall_status(test_counts.get_status())
 	return KunitResult(status=kunit_status, result=result.result, elapsed_time=exec_time)