Convert string formatting tests to be data driven

Summary:
Less boilerplate code in the actual test data.

Depends on D20031

Test Plan: LANG=C ctest

Reviewers: Baloo, Frameworks, ngraham, astippich, apol

Reviewed By: apol

Subscribers: mgallien, apol, kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D20032