[EngineQuery] Provide toString(Term) overload for QTest

Summary:
Without the overload QTest only provides the generic "Compared values
are not the same" message, without any details.

Also make the debug output more compact.

Reviewers: #baloo, ngraham

Reviewed By: #baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: #frameworks, #baloo

Differential Revision: https://phabricator.kde.org/D28505