[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.

autotests/integration/screenedge_client_show_test.cpp

@@ -164,6 +164,9 @@ void ScreenEdgeClientShowTest::testScreenEdgeShowHideX11()
     QVERIFY(!client->isHiddenInternal());
     QCOMPARE(effectsWindowShownSpy.count(), 1);
 
+    // go into event loop to trigger xcb_flush
+    QTest::qWait(1);
+
     //hide window again
     Cursor::setPos(QPoint(640, 512));
     xcb_change_property(c.data(), XCB_PROP_MODE_REPLACE, w, atom, XCB_ATOM_CARDINAL, 32, 1, &location);