tests: add test-compositor

This patch introduces a set of functions that can create a display
and clients for tests.
On server side the user can use functions:
  display_create()
  display_destroy()
  create_client()
  display_run()
  display_resume()
and on client side the user can use:
  client_connect()
  client_disconnect()
  stop_display()

The stop_display() and display_resume() are functions that serve as a barrier
and also allow the display to take some action after the display_run() was called,
because after the display is stopped, it can run arbitrary code until it calls
display_resume().

client_connect() function connects to wayland display and creates a proxy to
test_compositor global object, so it can ask for stopping the display later
using stop_display().

An example:

  void
  client_main()
  {
        /* or client can use wl_display_connect(NULL)
         * and do all the stuff manually */
        struct client *c = client_connect();

        /* do some stuff, ... */

        /* stop the display so that it can
         * do some other stuff */
        stop_display(c, 1);

        /* ... */

        client_disconnect(c);
  }

  TEST(dummy_tst)
  {
       struct display *d = display_create();

       /* set up the display */
       wl_global_create(d->wl_display, ...);

       /* ... */

       create_client(d, client_main);
       display_run();

       /* if we are here, the display has been stopped
        * and we can do some code, i. e. create another global or so */
       wl_global_create(d->wl_display, ...);

       /* ... */

       display_resume(d); /* resume display and clients */

       display_destroy(d);
  }
tests: use test compositor in queue-test

Most of the code of queue-test is covered by the test compositor,
so we can save few lines and use the test compositor
instead. I think it's also more readable.
tests: test posting errors

Test posting errors to one and more clients.