Eliminate use of `extract()` in `wp-activate.php`.

Props jeremyfelt, MikeHansenMe.
See #22400.

Built from https://develop.svn.wordpress.org/trunk@28373


git-svn-id: http://core.svn.wordpress.org/trunk@28201 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Eliminate use of `extract()` in `wp_insert_category()`. Adds unit tests. All unit tests pass.

Props MikeHansenMe.
See #22400.

Built from https://develop.svn.wordpress.org/trunk@28374


git-svn-id: http://core.svn.wordpress.org/trunk@28202 1a063a9b-81f0-0310-95a4-ce76da25c4cd