tests: very naive mockup of app.run_service
tests: very naive mockup of app.run_service

Currently it doesn't check passed arguments, does not allow specify
expected stdout/stderr or even return code.
It's only enough to not crash tests on NotImplementedError.

marmarek/qubes-core-mgmt-client,marmarek/qubes-core-mgmt-client,marmarek/qubes-core-mgmt-client