In that file there is a variable named message_params that is initialized with about 15 different sets of test data, in class TestEmailMessageBase, but is never referenced again. I even grepped for the variable name in all test py files to confirm that it isn't somehow imported somewhere.

Is there something about the test framework that uses that automatically? I'm hesitant to add any testing code for my issue if I'm missing something so fundamental.

Have a look at the "parameterize" decorator (in __init__.py in the same directory). It looks for things that .endswith('_params').