add gortsplib sps tests

[Curid]

This is all the gortsplib sps tests if you want them.

[dholroyd]

I think apart from the tests, the only non-formatting change is the addition of the fps() method, right? (In future consider putting the rustfmt changes in a separate commit for easier review - and yeah I confess I should have run rustfmt myself - and enforce it in CI!)

[Curid]

Sorry about the formatting. I can make two separate PRs for cargo fmt and cargo clippy first if you want.

gortsplib has a fps() method and tests for it. I don't think I have a use for it personally, would you like me to remove it?

[dholroyd]

I am happy to keep fps() in this PR.