tests: Set a dummy XDG_CONFIG_HOME variable

This avoids a local ~/.config/monitors.xml messing up tests.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2985>

src/tests/meson.build

@@ -137,6 +137,7 @@ test_env = environment()
 test_env_variables = {
   'G_TEST_SRCDIR': join_paths(mutter_srcdir, 'src'),
   'G_TEST_BUILDDIR': mutter_builddir,
+  'XDG_CONFIG_HOME': join_paths(mutter_builddir, '.config'),
   'MUTTER_TEST_PLUGIN_PATH': '@0@'.format(default_plugin.full_path()),
 }