New facility for deterministic and reproducible test PRNGs.

The new testing_rng() function is meant as a replacement for thread_rng() for use in unit tests. By default, it uses a randomly seeded RNG, but prints the seed before the test so that you can reproduce any failures that occur. You can override this via the environment to use a previous seed, or by using a deterministic seed for all your tests.

Backend for #486 (closed).