Without 'wayland/surface-actor: Reset and sync subsurface state when
resetting' this test would fail.
This also adds a simple framework for testing lower level Wayland
semantics.
In contrast to the test-client and test-driver framework, which uses
gtk and tests mostly window management related things, this framework is
aimed to run Wayland clients made to test a particular protocol flow,
thus will likely consist of manual lower level Wayland mechanics.
A private protocol is added in order to help out clients do things they
cannot do by themself. The protocol currently only consists of a request
meant to be used for getting a callback when the actor of a given
surface is eventually destroyed. This is different from the wl_surface
being destroyed due to window destroy animations taking an arbitrary
amount of time. It'll be used by the first test added in the next
commit.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/961
