[autotests] Try fixing TestWaylandOutputManagement

The test has KWayland::Client objects like Registry as member variables
of the test object. This causes the objects to be destroyed with the
dtor after cleanupTestCase is run which destroys the connection and
Wayland server. At least on the CI system this seems to cause problem.

In general our tests do not keep any state around, especially not
KWayland::Client objects. The normal way is to have a new dedicated
client connection for every test method. This test doesn't follow this
approach at all.

In case that this change does not fix the test and still crashes on
build.kde.org the only option is to drop the test and replace it by a
new variant which follows the approach of other tests.