Improved Code Coverage Database3

[john-science]

Description

We have been having some code coverage spray, in that certain lines in our code are covered (or not) semi-randomly when the unit tests are run.

That makes trusting our code coverage for a given PR harder.

So, I am permanently adding some code coverage to one such offending line (I think, perhaps, the last such line in our test):

https://coveralls.io/builds/51401612/source?filename=armi%2Fbookkeeping%2Fdb%2Fdatabase3.py#L1313

EDIT: I also threw in some extra code coverage and test cleanup in this PR. Sorry. The purpose of this PR is still to fix the spray on that one line above, but since I was already in the tests, I couldn't help myself.

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Checklist

• This PR has only one purpose or idea.
• Tests have been added/updated to verify that the new/changed code works.

• The code style follows good practices.
• The commit message(s) follow good practices.

• The release notes are up-to-date with any bug fixes or new features.
• The documentation is still up-to-date in the doc folder.
• The dependencies are still up-to-date in setup.py.

[albeanth]

Great additions @john-science ! Hopefully this helps alleviate the (seemingly) random code coverage failures in CI.

I just have one simple comment to remove the declaration of an unused class variable.