Get rid of a TestCase class. (#20656)

Turns its members into new-style standalone test_* functions 
(and a couple of helpers).

The diff is large, but the change is purely mechanical: dedent the
functions and remove all references to self. The proof is that all
77 tests in this file pass after this change (I have verified
that no tests are missed by the pytest collector).

This is in preparation for integrating the rust options parser, which
may require adding test fixtures, which is not a feature available
to old-style class based tests.

Author: benjyw