test_storage: Fix flakiness in round to milliseconds test method

Prior to this commit, the tests would fail [1] for no good reason [2]. This fixes it.

• [1] https://jenkins.softwareheritage.org/job/DSTO/job/tests/1264/console

• [2] microseconds would exceed a limit of 999999 from time to time

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Migrated from D3302 (view on Phabricator)