tests: Isolate basic test DBus session

Open Iñigo Martínez requested to merge inigomartinez/gnome-photos:isolate-tests into master

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)

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