test: More integration tests for rate limits

[jprenken]

The ratelimits package has excellent test coverage, but we have relatively few tests that check rate limit behaviour end-to-end. Let's add more cases to test/integration/ratelimit_test.go.

For example, although ratelimits/transaction_test.go tests that transactions get created as specified, we don't necessarily test that those functions' callers are producing the set of transactions we expect them to.