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

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

BugLink: https://bugs.launchpad.net/bugs/1812352
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>
(cherry picked from commit 3f4435b5149372b3bbc5acab5c835d490490d6bc)
Signed-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Acked-by: Andrea Righi <andrea.righi@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>