tests: Make the test backend derive from the native backend

It's constructed as a native backend headless backend, but uses a custom
monitor manager (based on MetaMonitorManagerNative) that creates a fake
monitor. Rendering is unconditionally done with the surfaceless
renderer.

The test devices used now use virtual devices, meaning some changes to
the tests to e.g. not set names, and not dealing with input devices
directly.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3812>

src/tests/stage-view-tests.c

@@ -1365,7 +1365,7 @@ main (int argc, char *argv[])
 
   g_setenv ("MUTTER_DEBUG_DISABLE_ANIMATIONS", "1", TRUE);
 
-  context = meta_create_test_context (META_CONTEXT_TEST_TYPE_NESTED,
+  context = meta_create_test_context (META_CONTEXT_TEST_TYPE_TEST,
                                       META_CONTEXT_TEST_FLAG_TEST_CLIENT);
   g_assert (meta_context_configure (context, &argc, &argv, NULL));