Check safety of service befores starting it inside `Testbed.start_service()`

[ehpor]

Currently, safety is only checked after the service process itself has started. If the testbed is unsafe and the service requires safety, the service will immediately shut down. We can do this check inside the testbed for even starting the process for extra safety.

[raphaelpclt]

Would be nice to have a 2nd layer of security. But the testbed would have to communicate with a service while opening a service, which would end up in a deadlock at the tcp level. Very involved.