tests: add a comment that we're purposely testing py2 extension attributes Avoid someone unknowingly removing test coverage. There are tests for a properly byteified `testedwith` a few lines down. I don't see similar for `buglink`, but it's a trivial conversion to bytes, so I'm not concerned about testing the expected/wanted extension state. Differential Revision: https://phab.mercurial-scm.org/D9434