Refactor Ops tests to use pytest instead of unittest

[benhoyt]

We've been moving to pytest for Harness and other examples in our docstrings and docs. However, the Ops tests themselves are still all written in unittest with ugly self.assertEqual(...) calls and the like. Let's go all in on pytest here too.

This is going to be a huge diff -- maybe we can do 1 file per PR to split it up?

Let's look to see if there are existing tools that do this. If not, we may want to consider writing our own AST-based hack that can do most of it.