[ffmpegextractor] Add Matroska Video test case

Summary:
The test file was generated by converting the webm video file, using:
$> ffmpeg -i test.webm -acodec copy -vcodec copy test.mkv

Depends on D19845

CCBUG: 403902

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/D19846