The original test_ext_alarms_replaces test was sensitive to system loads
that slowed down the execution of the test, resulting in intermittent
failures.
This new test tests the equivalent behavior (namely that calling
browser.alarms.create replaces a previous alarm), without being
sensitive to system load. This is achieved by creating a repeating alarm
and immediately replacing this alarm with a one-shot alarm that runs
later than the original alarm.
Differential Revision: https://phabricator.services.mozilla.com/D4698
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extra : moz-landing-system : lando
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