test_comparedb3.py has very few unit tests

[opotowsky]

It has a few small unit tests, and two integration tests that don't touch all possible cases. Unit tests for all the other functions are needed.

This was discovered when the need for PR #786 was discovered.

[john-science]

We have raised the code coverage from 60% to 83%:

https://coveralls.io/builds/51061594/source?filename=armi%2Fbookkeeping%2Fdb%2FcompareDB3.py

I think most of what is left are exceptions and errors. Which we have not done a lot of testing for yet in ARMI. Sadly.