kunit: Support skipped tests

The kunit_mark_skipped() macro marks the current test as "skipped", with
the provided reason. The kunit_skip() macro will mark the test as
skipped, and abort the test.

The TAP specification supports this "SKIP directive" as a comment after
the "ok" / "not ok" for a test. See the "Directives" section of the TAP
spec for details:
https://testanything.org/tap-specification.html#directives



The 'success' field for KUnit tests is replaced with a kunit_status
enum, which can be SUCCESS, FAILURE, or SKIPPED, combined with a
'status_comment' containing information on why a test was skipped.

A new 'kunit_status' test suite is added to test this.

Signed-off-by: David Gow <davidgow@google.com>
Tested-by: Marco Elver <elver@google.com>
Reviewed-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>