AS-260: Change max manifest testing to batch API calls

Retrospectively, this likely should have been in multiple PRs.
Non-blocking, since it's test infrastructure and Python doesn't really stop you from doing it this way, but... It may be a good idea for testability and general API to separate out the creation of an intent to do something and actually making some side-effectful post requests. @radix wrote [radix/effect](https://github.com/radix/effect) to do exactly this.