[autotests] Test desktop switching animations

Summary:
The new test verifies some common things about desktop switching
animation effects, it doesn't verify what each of one them is doing
because it wouldn't be worth it.

Test Plan:
    dbus-run-session ctest -R testDesktopSwitchingAnimation

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18156