When debugging modifier_only_shortcut_test in _waylandonly mode I saw
that it was failing, among other things, because some aspects were not
initialised.
This changes every test we have to run the new
Test::initWaylandWorkspace() that calls waylandServer()->initWorkspace()
but also makes sure that WaylandServer::initialized is emitted before we
proceed.
At the moment, the Screens class is convoluted with ifdefs because of
MockScreens.
The goal of this change is to reduce the number of usages of the
MockScreens class so it is possible to get rid of the ifdefs.