We check that we get surface.enter_output and move the pointer into
the window and make sure we get input_device.pointer_enter with
the right coordinates.

There's a lot of code for a very simple test here, so we need to
figure out how to reuse most of the event handling and such.  It's also
not clear that a custom, text based protocol is practical here, we might
just use a wayland extension after all.