tests: Isolate basic test DBus session
gnome-photo's basic test make use of the running DBus session. However, gnome-photo might not be installed and the service file not present.
The DBus session has been changed to be run under a new session created by using
dbus-run-session with a custom configuration where the gnome-photo binary and the service file will be located under the build directory.
Also see !48 (closed)