selftests: kselftest_harness: let PASS / FAIL provide diagnostic

Switch to printing KTAP line for PASS / FAIL with ksft_test_result_code(),
this gives us the ability to report diagnostic messages.

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>