Remove `Random.seed!` from test suite

[penelopeysm]

0. Global state bad
1. If it doesn't matter what the result is, don't set a seed.
2. If it just matters that the seed is reproducible but it doesn't matter what it generates, use Xoshiro.
3. If the exact output matters, use StableRNG.

[mhauru]

I would like to keep the single call to Random.seed! in runtests.jl. It indirectly sets the global RNG seed for all tests, because @testset sets the seed at the beginning and restores it at the end (so that you can reorder test sets without changing results). I agree that in all tests where this matters a lot we should explicitly set RNGs in things like calls to sample, using e.g. StableRNG. However, we will forget to do that at times, or think it doesn't matter somewhere when it actually does, and having that one line in runtests.jl is an extra level of protection that can save us in those cases from indeterministic test failures.

[penelopeysm]

Oh, sure! I was only thinking of the ones inside testsets.

[mhauru]

Oh yeah, those should go, I agree.