refactor tests for taglibextractor

Summary:
refactors all tests so that they can be reused for
files of different mimetype, but with the same metadata format

Reviewers: mgallien, bruns, svuorela

Reviewed By: bruns

Subscribers: kde-frameworks-devel, Baloo

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D16135