dbus-runner: Add colord mocking

This will be needed for adding colord integration without breaking
testing.

The test context is altered to make sure any left over color devices are
cleaned up before starting. This means it becomes possible to run a test
case multiple times without having to restart meta-dbus-runner.py.

Note: Don't use os.getlogin() to get the current username; as that
requires a controlling terminal.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2141>

src/tests/meta-backend-test.c

@@ -67,11 +67,6 @@ meta_backend_test_init_gpus (MetaBackendX11Nested *backend_x11_nested)
   meta_backend_add_gpu (META_BACKEND (backend_test), backend_test->gpu);
 }
 
-static void
-meta_backend_test_init (MetaBackendTest *backend_test)
-{
-}
-
 static MetaMonitorManager *
 meta_backend_test_create_monitor_manager (MetaBackend *backend,
                                           GError     **error)
@@ -175,6 +170,11 @@ meta_backend_test_remove_device (MetaBackendTest    *backend_test,
   clutter_event_free (event);
 }
 
+static void
+meta_backend_test_init (MetaBackendTest *backend_test)
+{
+}
+
 static void
 meta_backend_test_class_init (MetaBackendTestClass *klass)
 {