Dedicated service test method

This merge request adds the possibility to provide an implementation for the "Test service" button independently from the service's push hook. If the test method is not overridden by the service, it falls back to the previous behavior by calling the execute method.

This separation is especially useful for services which do not react on push events but whose setup should still be testable (e.g. external issue trackers). With this change, unnecessary requests made after each push can be avoided.