Make the RNG for tests reproducible

[mpenciak]

This issue aims to change all of the tests to use reproducible, seeded random values.

The following rng setup was recommended by @huitseeker:

use rand_chacha::ChaChaRng;
use rand_core::SeedableRng;

let mut rng = ChaChaRng::from_seed([0u8; 32]);

[storojs72]

Yes. Probably, this makes sense to have all test vectors using deterministic input/output