Date/Time: XML-RPC: Сalculate the proper offset for GMT in `wp.newPost`,...

Date/Time: XML-RPC: Сalculate the proper offset for GMT in `wp.newPost`, `wp.editComment`, `mw.newPost`, `mw.editPost` when `post_date` or `comment_date` is set.

Previously, `post_date` or `comment_date` was assumed to be GMT, which is only true if the timezone string for the site matches GMT.

Add unit tests.

Props Rarst, smerriman, justdaiv, wonderboymusic, noyle.
Fixes #30429.
Built from https://develop.svn.wordpress.org/trunk@46864


git-svn-id: https://core.svn.wordpress.org/trunk@46664 1a063a9b-81f0-0310-95a4-ce76da25c4cd