Refactor throttle middleware tests as prone to intermittent failure

https://github.com/go-chi/chi/blob/master/middleware/throttle_test.go

It's often that when I submit a PR, and the code runs through github ci, it will fail -- https://github.com/go-chi/chi/actions

I have to re-run the jobs to have it pass, it seems to be a timing issue. When running locally, it seems to work well pretty consistently, but I've seen the rare failing case.

Start with middleware/throttle_test.go and review the intermittent failures in the GitHub Actions runs linked in the issue. Reproduce the timing-sensitive behavior locally or in CI, then refactor the tests so the existing throttle coverage passes consistently without reruns.

Written by the indexing model from the issue text.