This change contains various changes and refactorings which together
allows the integration tests to pass again.
One of the important changs is `DbusTestHelper` which listens to a DBus
signal for a sentinel message and then simulates the sentinel key press.
Getting this to work also requires processing sd-bus messages which hooks
into the glib event loop and is implemented in `EventSource`.
Fixes: #13204
This change fixes#13171 where we failed to output after the first keypress
because the keyman system service had to be started. This change introduces
a new method `Ping` that we call to force keyman-system-service to be
started.
Fixes: #13171
