window/wayland: Constrain sporadic client resizes
When a client resizes on its own, make sure the new size is passed through the window constraints machinery, to trigger any potential window management rule that might apply.
Fix a couple of tests to make use of this behavior by introducing a new 'wait_size' command that waits until a window has been resized to a expected size.
Not sure this fixes any reported issues, just something I ran into while working on the test suite.