Fix selection and seat auto tests

Summary:
The seat test is broken since 5e195c2a67ce, because a frame event now
also follows a relative pointer motion.

The selection test is broken since 0e580ae9bdc5, because clearing a selection,
which never has been set, is a noop now.

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D14183