Fix selection and seat auto tests

Summary:
The seat test is broken since 312298e2, because a frame event now
also follows a relative pointer motion.

The selection test is broken since d3f15a53, 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