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

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

BugLink: https://bugs.launchpad.net/bugs/1812352
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>
(cherry picked from commit 42b44c34136857ccdf90ebb1cbc38f2bf0aec7a1)
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>