Automerge bot improvements: tests and paging

[balopat]

Description of the issue

There are a bunch of Github queries in dev_tools/auto_merge.py that are supposed to be paged - they haven't been failing as the PRs themselves are typically small, but weird behavior happens when the number of records is larger than the per_page number. We introduced a while ago get_all which does the paging automatically, we should introduce that to all of the queries.

Also, we should introduce testing against a fake Github API service implementation. There are precisely 0 tests written for it at the moment.