selftests: kselftest: change KSFT_SKIP=4 instead of KSFT_PASS

KSFT_SKIP points to KSFT_PASS resulting in reporting skipped tests as
Passed, when test programs exit with KSFT_SKIP or call ksft_exit_skip().
If tests are skipped because of unmet dependencies and/or unsupported
configuration, reporting them as passed leads to too many false positives.

Fix it to return a skip code of 4 to clearly differentiate the skipped
tests.

Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>