Use QSignalSpy::wait instead of QTest::wait where possible

Summary:
This allows the test to finish a bit earlier in the best case.
I.e. when the signal comes in after 1ms, we will only wait for 1ms,
instead of always waiting for 50ms.

Reviewers: dfaure

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D9821