selftests: lib.mk: add SKIP handling to RUN_TESTS define

RUN_TESTS which is the common function that implements run_tests target,
treats all non-zero return codes from tests as failures. When tests are
skipped with non-zero return code, because of unmet dependencies and/or
unsupported configuration, it reports them as failed. This will lead to
too many false negatives even on the tests that couldn't be run.

RUN_TESTS is changed to test for SKIP=4 return from tests to enable the
framework for individual tests to return special SKIP code.

Tests will be changed as needed to report SKIP instead FAIL/PASS when
they get skipped.

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