Need a regression test for layer shell surfaces submitting buffers while hidden

[tarek-y-ismail]

While working on #4664, I encountered a bug which can be reproduced with the following steps:

1. Run a layer shell surface in the "above" layer (waybar, ironbar, gbar)
2. Run a normal application
3. Fullscreen the normal application
4. Wait for some time
5. As the layer shell surface submits a new buffer, it will be shown in the middle of the screen vertically.

I've tried implementing this as a WLCS test in canonical/wlcs#405, but it does not fit in WLCS as different compositors will likely? treat this differently (if not, I'll happily reopen the PR).

I could not find a part of our testing infrastructure that allows for testing this. For example, window management tests do not exercise the path that triggers this bug:

mir/src/server/frontend_wayland/window_wl_surface_role.cpp

Lines 351 to 360 in bd22724

	bool const is_mapped = scene_surface->visible();
	bool const should_be_mapped = static_cast<bool>(surface.value().buffer_size());
	if (!is_mapped && should_be_mapped && scene_surface->state() == mir_window_state_hidden)
	{
	spec().state = mir_window_state_restored;
	}
	else if (is_mapped && !should_be_mapped)
	{
	spec().state = mir_window_state_hidden;
	}