[autotests] Fix ScreenEdgeClientShowTest::testScreenEdgeShowHideX11 on new CI

On the new CI system this test is failing. This might be a variation of
Qt 5.10 as I'm able to reproduce with Qt 5.10. From the investigation it
is not a bug in KWin. The test was failing due to a missing xcb_flush.
This happens when we go back to the event loop which seems not to happen
in proper timing. By adding a qWait we force to go to the event loop, the
flush is performed and the test passes.