No validation for $post_date_gmt parameter

[paulbonneau]

This PR aim to :

• implement a validation for $post_date_gmt in wp_insert_post equivalent to the existing validation for $post_date : as for $post_date validation, it interrupt the execution of wp_insert_post and return 0 or a WP_Error object if the date string isn't valid.
• refactor and abstract part of wp_resolve_post_date
• Update the tests to take into account the change in the principle of handling erroneous dates for post_date_gmt

Trac ticket: https://core.trac.wordpress.org/ticket/36738

[github-actions]

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[github-actions]

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[github-actions]

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