mock_dbus_monitor: make the path check optional
mock_dbus_monitor: make the path check optional

Since commit 2fbc3dc875149000b "MockDBusMonitor: raise exceptions if
unregistered paths are used", paths will no longer be created automatically.
This breaks the generator-start-stop units test, since it depends on that
behaviour. So make this check optional.

victronenergy/velib_python