test_ChangeDisplaySettingsEx() can generate so many posted messages
for display change events that USERPostMessageLimit is reached and
ERROR_NOT_ENOUGH_QUOTA is returned for PostMessage(). Call flush_events()
after mode changes to limit message post rate.

Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=48586Signed-off-by: Zhiyi Zhang <zzhang@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>