selftests: kselftest: Add ksft_test_result_xpass

The functions ksft_test_result_pass, ksft_test_result_fail,
ksft_test_result_xfail, and ksft_test_result_skip already exist and are
available for use in selftests, but no XPASS equivalent is
available.

This adds a new function to that family that outputs XPASS, so that it's
available for future test writers.

Link: https://lore.kernel.org/r/20241207012325.56611-1-me@steffo.eu


Signed-off-by: Stefano Pigozzi <me@steffo.eu>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>