functional-tests: detect-h264-codec.sh looks for gstreamer plugins even if tracker-miners built to use libav
tests/functional-tests/meson.build calls detect-h264-codec.sh even if gstreamer isn't the chosen generic media extractor, but libav is. This doesn't make sense in this case, as h264 decoding capability depends on ffmpeg then, not gstreamer installed plugins. I'm not sure if libav/ffmpeg could be built to not support h264 (but probably due to patents?), so not sure if a check is needed in libav case, or we could assume it will always work.
Additionally it would be nice to tell the build system if to run them or not, overriding the automatic detection, due to sandboxed build having issues with gst-inspect in case v4l2 or similar plugins are installed (accessing /dev/dri stuff, which sandbox denies, and gst-plugins-scanner and the whole build process gets killed by my sandbox). But that I handled downstream right now and is a separate issue.