Tests for nova error injection 

addresses #73
Adding the appropriate calls to `clock` should be very easy, since we already have a clock on the mimic core.  Given that it should only be a few minutes of work, putting `time.sleep()` into tests to avoid it seems really bad.

If we want to have stuff like this - temporary fixes - I think the process should be:

  1. file an issue to fix it, linking to the PR you were working on
  2. put the link to the issue into the comment

that way there is some persistent record of the work we need to do and it doesn't just silently cause problems (for example, slowing down the test suite) forever.