rate_limiter: Fix flakiness of tests in test_external.
rate_limiter: Fix flakiness of tests in test_external.

I noticed RateLimitTests.test_hit_ratelimits fails when run as an
individual test, but never when run after other tests. That's due to the
first API request in a run of tests taking a long time, as detailed in
the comment on the change to the setUp method.

punchagan/zulip,hackerkid/zulip,punchagan/zulip,showell/zulip,punchagan/zulip,zulip/zulip,andersk/zulip,andersk/zulip,showell/zulip,andersk/zulip,hackerkid/zulip,showell/zulip,eeshangarg/zulip,kou/zulip,kou/zulip,punchagan/zulip,zulip/zulip,showell/zulip,hackerkid/zulip,hackerkid/zulip,kou/zulip,zulip/zulip,eeshangarg/zulip,hackerkid/zulip,rht/zulip,showell/zulip,showell/zulip,punchagan/zulip,andersk/zulip,eeshangarg/zulip,punchagan/zulip,andersk/zulip,eeshangarg/zulip,eeshangarg/zulip,kou/zulip,hackerkid/zulip,punchagan/zulip,zulip/zulip,hackerkid/zulip,eeshangarg/zulip,andersk/zulip,zulip/zulip,zulip/zulip,zulip/zulip,rht/zulip,rht/zulip,kou/zulip,kou/zulip,kou/zulip,andersk/zulip,rht/zulip,eeshangarg/zulip,rht/zulip,rht/zulip,rht/zulip,showell/zulip