[ffmpegextractor] Convert test to be data driven

Summary:
Make it easier to extend to more file types, no
functional or coverage change (yet).

Test Plan: ctest

Reviewers: Baloo, Frameworks, astippich, mgallien, ngraham

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D19845