Verify that `human_time_diff` works as expected for minute, hour and day intervals. Test that rounding works as expected when the time difference is near the rounding margin.
Props SGr33n.
Fixes#38773.
git-svn-id: https://develop.svn.wordpress.org/trunk@41018 602fd350-edb4-49c9-b593-d223f7449a82
