[autotest] Extend test to verify the code which handles buffer deletions

Summary:
Surface/SubSurface and Shadow handle the case that an attached buffer
gets destroyed by the client. So far we didn't have this code covered,
but it's rather important as incorrect reference counting can hit
asserts.

Reviewers: #plasma

Subscribers: plasma-devel

Projects: #plasma

Differential Revision: https://phabricator.kde.org/D1403