[3.12] GH-113661: unittest runner: Don't exit 5 if tests were skipped (GH-113856) (#113875)

GH-113661: unittest runner: Don't exit 5 if tests were skipped (GH-113856)

The intention of exiting 5 was to detect issues where the test suite
wasn't discovered at all. If we skipped tests, it was correctly
discovered.
(cherry picked from commit 3a9096c337c16c9335e0d4eba8d1d4196258af72)

Co-authored-by: Stefano Rivera <stefano@rivera.za.net>