They are a thing [1]. We use MintData spreadsheets for a large part of our regression test suite that tests MintData itself.
A bit meta, but yes, spreadsheets can definitely be used for the equivalent of what "unit tests" are in code.
Interestingly, we also have "integration tests" in our spreadsheets, but this has more to do with the fact that MintData spreadsheets have native API calling ability, so we can test with external services end-to-end.
