tests: Test whether a simple EGL main loop uses too many buffers

tests: Test whether a simple EGL main loop uses too many buffers

This adds a test that tries to simulate a simple game loop that would
be like this:

while (1) {
        draw_something();
        eglSwapBuffers();
}

In this case the test is relying on eglSwapBuffers to throttle to a
sensible frame rate.

The test then verifies that only 2 EGL buffers are used. This is done
via a new request and event in the wayland-test protocol.

Currently this causes 3 buffers to be created because the release
event generated by the swap buffers is not processed by Mesa until it
blocks for the frame complete event in the next swap buffers call, but
that is too late.

This can be fixed in Mesa by issuing a sync request after the swap
buffers and blocking on it before deciding whether to allocate a new
buffer.