Run tst_QEventDispatcher with the GUI event dispatchers as well

Run tst_QEventDispatcher with the GUI event dispatchers as well

Since some GUI event dispatchers are complete reimplementations and do
not build on the corelib ones, we want to run the same tests with the
other dispatcher.

Since this is a GUI test now, we need to make sure to drain system
events queued during application startup to make sure we can reliably
run the test functions.

Change-Id: I4905db70bc8f8584c4ef1f4d767824040281452c
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>