Fix error in WaylandSurface::testDisconnect
This test deletes the client wayland connection.
We then finish the test and call test cleanup which
Calling release after the connection is destroyed is an error.
Hence the explicit destroy.
Built tests with ASAN
No more crash
Reviewers: Plasma, graesslin
Reviewed By: Plasma, graesslin
Subscribers: plasma-devel, Frameworks
Differential Revision: https://phabricator.kde.org/D9402