tests: Make sure XDG_CURRENT_DESKTOP is unset

GSettings overrides can be active and set the default value depending on
the XDG_CURRENT_DESKTOP environement variable. For the tests we run, we
rely on the default settings by using the GSettings memory backend but
we also need to make sure not overrides are in place.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3730>

src/tests/mutter_dbusrunner.py

@@ -265,6 +265,7 @@ def wrap_call(args, wrapper):
     env['NO_AT_BRIDGE'] = '1'
     env['GTK_A11Y'] = 'none'
     env['GSETTINGS_BACKEND'] = 'memory'
+    env['XDG_CURRENT_DESKTOP'] = ''
     env['META_DBUS_RUNNER_ACTIVE'] = '1'
 
     if wrapper == 'gdb':