Moves away from Committee and towards stripe-mock, an external
self-contained executable API stub server based on OpenAPI [1]. The
motivation here is that instead of making stripe-ruby a special
snowflake, we can use a single well-tested and feature-rich mock
implementation to drive every API's test suite.
[1] https://github.com/stripe/stripe-mock
Follows the path established in 2d75c8f by porting the rest of
stripe-ruby's tests over to OpenAPI. There are a few other changes here
where I've removed some tests that are duplicated or don't make much
sense, or reorganized how we test certain things, but this commit is
largely the same migration operation applied in bulk a few dozen test
suites.
