Allow auto-assertions to run even when `Mneme.start()` has not been called

[zachallaun]

See #21 for context.

• Improve the behavior of tests when they are run without Mneme.start() -- currently you get a "process is not alive" error.
• If the Mneme server wasn't started, run auto-assertions in non-interactive mode, which essentially means just run the assertion and fail without prompting.
• If someone prefers not to change the behavior of mix test, you can remove Mneme.start() from your test_helper.exs and make a custom mix mneme alias that starts Mneme before running your tests.