wp_update_post overwrites post_date when updating post_status

[SirLouen]

For this reviewed patch I've considered these elements:

1. I've taken as the fix: https://core.trac.wordpress.org/attachment/ticket/62468/fix-wp-update-post.patch
   It was unnecesary to preserving date of publish and private on draft/pending posts. Only future is relevant in this scenario

2. For the unit tests I've taken this one by @Sukhendu2002, removing all the extra AI fluff. The data provider feels a bit overkill but I think it will don't do any harm.

3. Finally, given that here is a good opportunity to open a specific file to cover wp_update_posts I've brought all the tests covering this function and merged them all into this file for Unit Tests organizational purposes.

Trac ticket: https://core.trac.wordpress.org/ticket/62468

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[peterwilsoncc]

I've added a few notes inline.

[SirLouen]

@peterwilsoncc as a rule of thumb, in a bug, I don't like to add tests that cannot be proven with and without the patch (like in TDD). But I've taken advantage of the dataProvider to add a number of cases, including those you mentioned (some are implicit, for example, a publish to future in the past, ending in publish as expected).