Date/Time: In `wp_insert_post()`, when checking the post date to set `future`...

Date/Time: In `wp_insert_post()`, when checking the post date to set `future` or `publish` status, use a proper delta comparison.

[3525] allowed a difference up to 59 seconds between the post date/time and the current time to consider the post published instead of scheduled, but that didn't take start of a new minute into account.

Rapidly creating post fixtures in unit tests could encounter a one-second discrepancy between `current_time( 'mysql' )` and `gmdate( 'Y-m-d H:i:s' )`, returning values like `2019-12-16 23:43:00` vs. `2019-12-16 23:42:59`, respectively, and setting the post to a `future` status instead of `publish`.

[45851], while working as intended, made the issue somewhat more likely to occur.

This caused all sorts of occasional random failures in various tests on Travis, mostly on PHP 7.1.

Fixes #48145.
Built from https://develop.svn.wordpress.org/trunk@46968


git-svn-id: https://core.svn.wordpress.org/trunk@46768 1a063a9b-81f0-0310-95a4-ce76da25c4cd