[tor-bugs] #28878 [Core Tor/Tor]: Improve deterministic randomness in tests

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#28878: Improve deterministic randomness in tests
-------------------------+-------------------------------------------------
     Reporter:  asn      |      Owner:  (none)
         Type:  defect   |     Status:  new
     Priority:  Medium   |  Milestone:  Tor: unspecified
    Component:  Core     |    Version:
  Tor/Tor                |   Keywords:  wtf-pad, tor-relay, tor-cell,
     Severity:  Normal   |  padding, tor-tests
Actual Points:           |  Parent ID:  #28637
       Points:           |   Reviewer:
      Sponsor:           |
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 As part of test_prob_distr.c in #28142, 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[32] 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.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/28878>
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