Example failure: https://buildkite.com/rails/rails/builds/82905#80d6c6ec-943d-4ba3-b360-1ef6c4aa5d89/1012-1022

The test designates the event end time as 0.01 seconds (i.e. 10
milliseconds) after the start time.  It then asserts that the event
duration is 10 ± 0.0001 milliseconds.  This sometimes fails due to
floating point precision errors.

This commit changes the assertion to instead check that the duration is
within 1% of the expected value.