Port test from rand_pcg to rand_chacha

[vks]

This would allow us to move rand_pcg to the rngs repository, see #1139.

[dhardy]

I'm not convinced this is a good idea: (1) there's very little actual incentive to move rand_pcg to the rngs repo, and (2) PCG is a much simpler RNG than ChaCha, thus if we ever decided to replace ChaCha we'd have to port tests again instead of just keeping a small RNG around for tests.

So my vote is to close this and forget about the issue.

[vks]

I don't have a strong opinion either way. @newpavlov What do you think?

[newpavlov]

I have a slight preference towards porting tests to rand_chacha, but I am fine with leaving everything as-is.

[dhardy]

Given the responses I'll close this. If more reasons come up we can reconsider.