Introduce seek test for Opus

[yvg]

@tiago-loureiro @tsenart any idea why Node is not serving the Opus file correctly?
It seems to think it is a binary file and does not stream it.

In the mean time I took the test out b73ec42

[tiago-loureiro]

I have seen that before and didn't get it fixed... ended up using nginx for the test. Needs further checking cc @ohookins

[ohookins]

Wasn't it just the mime type being unknown on the server? Or incorrect.

We stored the Opus files as .opus, but you guys are storing as .ogg so if the extension is being used, perhaps the server is assuming the container has vorbis content inside of it?