WTF-PAD: Improve deterministic randomness in tests

As part of test_prob_distr.c in legacy/trac#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[4] or something. c) Allow users to overwrite the seed using environment variables or the CLI. For now users have to tweak the init_deterministic_rand() func.