Track outputs in kwin integration tests

Summary:
Split from the last review, as in order to track output's removed (in
any way that's useful) the registry needs to be on the heap and the change grew.

Test Plan:
Every test that previously passed on my system still does
Though currently that's not every test (for some reason) which was
my main motivation for splitting.

Reviewers: #plasma, graesslin

Reviewed By: #plasma, graesslin

Subscribers: plasma-devel, kwin, #kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D7377