tests/stage-view: Don't compare old stage views to new ones on hotplug

The stage-view test currently segfaults sometimes in meta_test_actor_stage_views_hot_plug(), because items of the prev_stage_views list no longer exist.

That's because clutter_stage_peek_stage_views() returns a list owned by the stage, and this list is recreated when hotplugging monitors. Also the elements of this list, the stage views itself, might get freed when hotplugging a monitor.

That means we can't safely use this old list and its stage-views after a hotplug happened, so simply stop using it and don't compare the old views to the new ones.