Change logic of ktap parser to run on a thread

The ktap parser should be listening and parsing messages as the tests
are executed, and not after the end of module load.

v1 -> v2:
- fix coding style
- remove usleep
- add error check logic
- follow the structure of igt_kselftests more closely

v2 -> v3:
- fixed sublevel issues by rewriting tap parser flow

v3 -> v4:
- fixed delimiter

- squashed lib/igt_kmod: fix nesting igt_fixture in igt_subtest
Fix the following issue:
	$ ./build/tests/drm_buddy
	Starting subtest: all-tests
	nesting igt_fixture in igt_subtest is invalid
	please refer to lib/igt_core documentation

- squashed lib/igt_kmod: place KUnit tests on a subtest
There's a hidden bug at KUnit implementation: as it doesn't
place tests inside a subtest, trying to use it with igt_main
causes a crash:

	$ ./build/tests/drm_mm --list
	skipping is allowed only in fixtures, subtests or igt_simple_main
	please refer to lib/igt_core documentation
	drm_mm: ../lib/igt_core.c:437: internal_assert: Assertion `0' failed.
	Received signal SIGABRT.
	Stack trace:
	 #0 [fatal_sig_handler+0x17b]
	 #1 [__sigaction+0x50]
	 #2 [__pthread_kill_implementation+0x114]
	 #3 [gsignal+0x1e]
	 #4 [abort+0xdf]
	 #5 [__assert_fail_base.cold+0xe]
	 #6 [__assert_fail+0x47]
	 #7 [internal_assert+0xe5]
	 #8 [igt_skip+0x169]
	 #9 [__igt_skip_check+0x1bb]
	 #10 [igt_ktest_begin+0xa6]
	 #11 [igt_kunit+0x70]
	 #12 [main+0x2a]
	 #13 [__libc_start_call_main+0x7a]
	 #14 [__libc_start_main+0x8b]
	 #15 [_start+0x25]

  Fix it by using igt_subtests() before actually implememnting
  KUnit logic.

  After the patch, it should now report subtests:

	$ ./build/tests/drm_mm --list
	all-tests

Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>

Signed-off-by: Dominik Karol Piątkowski <dominik.karol.piatkowski@intel.com>
Acked-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>