Previously keyman-system-service didn't have any unit tests. With the
gcov/lcov versions available for Ubuntu 24.04 Noble this suddenly caused
a failure when trying to generate the test coverage report, although it
worked fine with older versions.
This change adds a do-nothing unit test stub for keyman-system-service.
This allows to generate the coverage reporting with Ubuntu 24.04 Noble.
Cherry-pick-of: #13740
