selftests: kselftest: add ksft_test_result_code(), handling all exit codes

For generic test harness code it's more useful to deal with exit
codes directly, rather than having to switch on them and call
the right ksft_test_result_*() helper. Add such function to kselftest.h.

Note that "directive" and "diagnostic" are what ktap docs call
those parts of the message.

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