WTF-PAD: Improve deterministic randomness in tests
As part of test_prob_distr.c in #28142 (moved), we implemented a deterministic randomness system for the stochastic tests. There are various things we can improve there:
a) Make it into its own subsystem so that other tests can also use it.
b) Don't use a uint32_t counter as the seed, because the byte order is machine dependent. Instead make it a
uint8_t or something.
c) Allow users to overwrite the seed using environment variables or the CLI. For now users have to tweak the