[tests] Install eventDispatcher before creating the QGuiApplication

This change demonstrates how one can install a custom event dispatcher
before creating the QGuiApplication and how to process Wayland events
using the event dispatcher to e.g. start an Xwayland server prior to
creating the application.

It also allows to use

--platform wayland

on the test application. The wayland QPA plugin will connect to the
KWayland server started prior to creating the application.
Please note that such a setup will freeze unless QtWayland has commit
1e32e71 [1].

[1] https://qt.gitorious.org/qt/qtwayland/commit/1e32e71403a6a9cb117343464fbc34564598e831