tests/monitor-unit-tests: Sleep some after each hot plug

Give clients (such as Xwayland) a chance to bind the wl_output global
before we continue, otherwise there is an significant risk that mutter
won't see the bind request until after the next hot plug which might
have destroyed the global object.

https://bugzilla.gnome.org/show_bug.cgi?id=730551

src/tests/monitor-unit-tests.c

@@ -954,6 +954,7 @@ emulate_hotplug (MetaMonitorTestSetup *test_setup)
     META_MONITOR_MANAGER_TEST (monitor_manager);
 
   meta_monitor_manager_test_emulate_hotplug (monitor_manager_test, test_setup);
+  g_usleep (G_USEC_PER_SEC / 100);
 }
 
 static void