o Minor features (testing):
    - The circuitpadding tests now use a reproducible RNG implementation,
      so that if a test fails, we can learn why. Part of ticket 28878.
    - Tor's tests now support an environment variable, TOR_TEST_RNG_SEED,
      to set the RNG seed for tests that use a reproducible RNG.
      Part of ticket 28878.

  o Minor features (continuous integration):
    - When running coverage builds on Travis, we now set TOR_TEST_RNG_SEED,
      to avoid RNG-based coverage differences.
      Part of ticket 28878.