d3d9/tests: Flush events after minimizing and restoring focus window.
Minimizing the window might trigger a focus change to the dummy window, while restoring and setting foreground should change the foreground back to the focus window.
However, the X window manager will instead send WM_TAKE_FOCUS event to the dummy window and to the focus window concurrently, and the dummy window might reply to it after we've restored the foreground to the focus window. This will later cause the device window to fail to get foreground, and the tests to randomly fail.
(This works around a foreground window race condition that will be difficult to fix otherwise, as it can happen in a cross-process context as well)