selftests: lib.mk: cleanup RUN_TESTS define and make it readable

Refine RUN_TESTS define's output block for summary and non-summary code
to remove duplicate code and make it readable.

cd `dirname $$TEST` > /dev/null; and cd - > /dev/null; are moved
to common code block and indentation fixed.

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