Fix ASAN warning on TestXdgForeign
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.
Compiled with ASAN
Reviewers: Plasma, graesslin
Reviewed By: Plasma, graesslin
Subscribers: plasma-devel, Frameworks
Differential Revision: https://phabricator.kde.org/D9406