Apply `order` to each passed value for `orderby` in `WP_Query`:

* Since `orderby` in `WP_Query` can accept space-delimited sets, yet only one `order` value: when multiple values are passed (and `DESC` is the order), the default sort order `ASC` is being applied to all values before the last in the set.
* There is a unit test that sporadically fails since 3.6 in `tests/post/revision` due to multiple posts having the same `post_date` from being added so rapidly
* When ordering revisions in `wp_get_post_revisions()`, order by `post_date ID`
* Change the `order` value in `wp_get_post_revisions()` to `ASC`. This will produce SQL like: `ORDER BY $wpdb->posts.post_date ASC, $wpdb->posts.ID ASC`. Previously, this would have produced SQL like: `ORDER BY $wpdb->posts.post_date DESC`, and with the addition of ` ID`: `ORDER BY $wpdb->posts.post_date ASC, $wpdb->posts.ID DESC`. Clearly, wrong. The original SQL produced: `ORDER BY $wpdb->posts.post_date DESC`. As such, return the reversions in reverse order using `array_reverse()`. Not doing so would break "Preview Changes."
* Add unit tests to assert that all of this works.
* All existing unit tests pass with the change to ordering multiple `orderby`s in `WP_Query`.
* In the future, we should support independent `order` for each `orderby`, see #17065.

Props SergeyBiryukov, wonderboymusic.
Fixes #26042.

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