test-ratelim: calculate timers bias (for slow CPUs) to avoid false-positive This can be/should be done for regression tests too. Refs: https://ci.appveyor.com/project/libevent/libevent/builds/28916689/job/kg621aa194a0qbym Refs: https://github.com/libevent/libevent/pull/917#issuecomment-553811834 v2: EVENT_BASE_FLAG_PRECISE_TIMER