Adding code coverage to snapshots and massFracs

[john-science]

Description

While I was bug hunting last week, I built some unit tests to make sure things were working. The branches I worked on didn't come to much, but I figure we can use the code coverage anyway.

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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 (location doc/release/0.X.rst) 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.

[opotowsky]

Nice to slip this in!