Resolve "timer_test unit test intermittently fails in CI"

Arаm Sаrgsyаn requested to merge 4285-timer_test-timer_event-fix into main

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)

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