Some tests reuse the same MP4 stream and are currently passing only because we have the generic byte stream handler fallback. They pass on Windows too, so I'm assuming the stream is seeked before looking for the hints. We could also very well remove the position restore because the media sources are rewinding the stream later too, but I've kept it like it was.
-- v2: mfplat: Seek byte stream to the start for URL hint detection. mfplat/tests: Test source resolver bytestream interactions.