`wp_schedule_single_event()` should not prevent scheduling a future duplicate event. It should only reject an event as a duplicate if there is already a similar event scheduled within 10 minutes of the given timestamp.

Adds unit tests, fixes existing cron test.

Props tellyworth.

See [9181], #6966.
Fixes #28213.


git-svn-id: https://develop.svn.wordpress.org/trunk@29939 602fd350-edb4-49c9-b593-d223f7449a82