Resolve "timer_test unit test intermittently fails in CI"
The resolution of the uv_hrtime() function is bigger than the intervals used in the timers, which can result in an unexpected difference between the start_time and stop_time variables.
Use isc_loop_now(), which is based on uv_now() and has the same milliseconds resolution as the functions in the uv_timer_t API.
Also fix a couple wrong numbers in the comments.
Closes #4285 (closed)