Determine a meaningful way to test `mut` params in function bodies

[ErichDonGubler]

The mutability test in the current test harness seems satisfactory for testing the parsing of the mut keyword in the macro we've specified. However, the test bodies themselves don't seem very meaningful, since most params are unused and thus force allow(unused) to avoid warnings. I want to figure out how to write a test that doesn't give the warning that mut isn't necessary, but haven't figured out how to do so yet.