[autotests] Make lifespan of EffectsHandler outlive Effect

Summary:
In loading tests. Effects are deleted in a deleteLater potentially
outside the scope of our test. Our MockEffectsHandler (which contains
the global static "effects") has the lifespan of the test.

Fixes failing unit test.

Test Plan: Ran test

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D15167