test: use reproducible input

vlc_stream_NewURL() can spawn stream filters which may alter the content or even the nature of the stream, which is not suitable here.

vlc_access_NewMRL() would avoid loading stream filters at all. But it would also skip testing the cache filter, thus missing much of the point of the test case.

So use a reproducible pseudorandom input using a constant seed, which is known not to trigger any stream filter.

Fixes #26569 (closed).