Fix use after free in tst_WaylandClient::dontCrashOnMultipleCommits

QBackingStore had a pointer to TestWindow window, but it was destroyed only
after the TestWindow was already deleted.

Change-Id: Id8fe38277cf843d532f2d8c2fdbe1f69d0309a7f
Reviewed-by: Johan Helsing <johan.helsing@qt.io>