Increase Unit Test Coverage

MESA now reports which lines of code are run and tested when we do an `./install` here: https://mesastar.org/mesa/

As can be seen, half of the functions are not tested by unit tests. This can be improved

Run `./install` and review the coverage report linked in the issue to identify untested functions. The issue does not name specific files or tests, so first determine which unit-test areas are in scope. Done means adding unit tests that exercise more of the currently uncovered code and confirming improved coverage.

Written by the indexing model from the issue text.