Isolate every test within WaylandOutputManagement

Summary:
The current code used the same outputInterface between all tests, with
data and outdated connections slowly accumulating.

This meant most the code didn't work as it was intended, for
example testExampleConfig had the config applied from the connect in the
previous test. It just happened to pass.

This resets everything between each test.

Test Plan:
Can now add a test without going insane
Existing tests still pass

Reviewers: #kwin, romangg

Reviewed By: #kwin, romangg

Subscribers: romangg, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D13729