button-test: Use wl_display_roundtrip instead of yield()

A round trip is sufficient here.  We need to make sure that the server
has received the wl_test request and that we've received the event
that the request triggers.  The wl_display_roundtrip() helper does
exactly that: it sends a wl_display.sync request, which will hit the
server after the wl_test requests and thus the wl_callback.done event
will come back after the server has seen all the previous requests and
after we've handled all preceeding event.

tests/button-test.c

@@ -42,13 +42,13 @@ TEST(simple_button_test)
 
 	wl_test_send_button(client->test->wl_test, BTN_LEFT,
 			    WL_POINTER_BUTTON_STATE_PRESSED);
-	yield(client);
+	wl_display_roundtrip(client->wl_display);
 	assert(pointer->button == BTN_LEFT);
 	assert(pointer->state == WL_POINTER_BUTTON_STATE_PRESSED);
 
 	wl_test_send_button(client->test->wl_test, BTN_LEFT,
 			    WL_POINTER_BUTTON_STATE_RELEASED);
-	yield(client);
+	wl_display_roundtrip(client->wl_display);
 	assert(pointer->button == BTN_LEFT);
 	assert(pointer->state == WL_POINTER_BUTTON_STATE_RELEASED);
 }