Fix testActivities

Currently, the test passes because the activities controller doesn't
have enough time to fetch all activities. So, the test client won't be
placed on the current activity.

Since the Workspace is now created before starting the Xwayland, we now
spin the event loop, which allows the activities controller to fetch the
list of all activities and so the test client will be placed on the
current activity, i.e. client->isOnAllActivities() will no longer return
true.

This change fixes wrong assumptions in the test and makes it robust.