Fix and test libav code paths
We "support" using libav instead of gstreamer for 'generic' media extraction, but we don't actually test that code path at all in CI.
Here's a plan to fix it:
- Add libav / ffpmpeg to the container build steps in gitlab-ci.yml
- Modify gitlab-ci.yml to add an extra job that configures tracker-miners with
-Dgeneric_media_extractor=libav
, runs the functional test suite again