Fix ASAN warning on TestXdgForeign

Summary:
Internally XdgForeign has some delete laters

If we clean up the connection immediately we delete them after we kill
the connection resulting in an error.

Kwin isn't going to tear down the connection in response to a resource
unbinding so we can just fix the test.

Test Plan:
Compiled with ASAN
Got further

Reviewers: #plasma, graesslin

Reviewed By: #plasma, graesslin

Subscribers: plasma-devel, #frameworks

Tags: #frameworks, #plasma

Differential Revision: https://phabricator.kde.org/D9406