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.

GNOME/gnome-photos!48
3 jobs for isolate-tests in 13 minutes and 12 seconds (queued for 4 seconds)
latest
Status Job ID Name Coverage
  Test
passed #187977
flatpak

00:12:30

 
  Review
passed #187978
review

00:00:42

manual #187979
allowed to fail manual
stop_review