selftests: lib.mk: move running and printing result to a new function

RUN_TESTS function has grown and becoming harder to maintain. Move
the code that runs and tests for returns codes to a new function
and call it from RUN_TESTS.

A new RUN_TEST_PRINT_RESULT is created to simplify RUN_TESTS and make it
easier to add handling for other return codes as needed.

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