[Term] 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.

Reviewers: Baloo, ngraham

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D28393