[autotests] Fix maximize test

Summary:
testMaximizedPassedToDeco was failing because it didn't handle async
maximization.

testBorderlessMaximizedWindow was failing because setNoBorder can modify
geometry, so we end up with a wrong restore geometry.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D16755