Fix error in WaylandSurface::testDisconnect

Summary:
This test deletes the client wayland connection.

We then finish the test and call test cleanup which
releases m_idleInhititManager.
Calling release after the connection is destroyed is an error.

Hence the explicit destroy.

Test Plan:
Built tests with ASAN
No more crash

Reviewers: #plasma, graesslin

Reviewed By: #plasma, graesslin

Subscribers: plasma-devel, #frameworks

Tags: #frameworks, #plasma

Differential Revision: https://phabricator.kde.org/D9402