Administration: Avoid a PHP 7.4 notice in `add_meta_box()` when attempting to...

Administration: Avoid a PHP 7.4 notice in `add_meta_box()` when attempting to re-add a previously removed box.

The logic for skipping previously removed meta boxes with the `core` priority should also apply to the `sorted` priority that is used when the boxes were manually reordered.

Add a unit test.

Props coolmann, franzarmas, SergeyBiryukov.
Fixes #50019.
Built from https://develop.svn.wordpress.org/trunk@47777


git-svn-id: https://core.svn.wordpress.org/trunk@47553 1a063a9b-81f0-0310-95a4-ce76da25c4cd