Fix unstable query test for post tags

[flixos90]

The Tests_Query_Results::test_query_tag__not_in() test is unstable and thus may occasionally fail, entirely random, see for example ​https://github.com/WordPress/wordpress-develop/actions/runs/7573278489/job/20625142895?pr=5658.

This is because two of the posts that are generated have the exact same date and time and thus their order in the query result is random.

For the same reason, the other test methods test_query_paged() and test_query_paged_and_posts_per_page() in the same class are unstable as well.

An easy fix for this is to use unique dates.

It looks like this goes back to the almost 10 year old commit [30276]. :)

This PR ensures the post dates are distinct, to avoid randomness in the results order.

With this, the many-trackbacks and no-comments test posts no longer use the same date.

Trac ticket: https://core.trac.wordpress.org/ticket/60288

Change looks good to me. Sorry for the trouble :-D

Committed in https://core.trac.wordpress.org/changeset/57308 - for some reason not properly showing up in SVN yet.

[flixos90]

Fixed via [57308] (somehow not showing in SVN, but it's there).