Eliminate use of `extract()` in `wp_dropdown_pages()`. 

Adds unit tests to: `tests/post/template.php`. 
There was previously only one wimpy assertion for `wp_dropdown_pages()`.
	
See #22400.

Built from https://develop.svn.wordpress.org/trunk@28399


git-svn-id: http://core.svn.wordpress.org/trunk@28227 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Eliminate use of `extract()` in `wp_list_pages()` which, surprisingly, didn't even use any of the extracted variables.
	
See #22400.

Built from https://develop.svn.wordpress.org/trunk@28401


git-svn-id: http://core.svn.wordpress.org/trunk@28228 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Eliminate use of `extract()` in `Walker_Page::start_el()`.
	
See #22400.

Built from https://develop.svn.wordpress.org/trunk@28402


git-svn-id: http://core.svn.wordpress.org/trunk@28229 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Eliminate use of `extract()` in `get_bookmarks()`.
	
See #22400.

Built from https://develop.svn.wordpress.org/trunk@28403


git-svn-id: http://core.svn.wordpress.org/trunk@28230 1a063a9b-81f0-0310-95a4-ce76da25c4cd