selftests: avoid using SKIP(exit()) in harness fixure setup

selftest harness uses various exit codes to signal test
results. Avoid calling exit() directly, otherwise tests
may get broken by harness refactoring (like the commit
under Fixes). SKIP() will instruct the harness that the
test shouldn't run, it used to not be the case, but that
has been fixed. So just return, no need to exit.

Note that for hmm-tests this actually changes the result
from pass to skip. Which seems fair, the test is skipped,
after all.

Reported-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/all/05f7bf89-04a5-4b65-bf59-c19456aeb1f0@sirena.org.uk
Fixes: a7247079

 ("selftests: kselftest_harness: use KSFT_* exit codes")
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Tested-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Link: https://lore.kernel.org/r/20240304233621.646054-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>