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

Adds unit tests to a new file: `tests/post/template.php`. 
There were previously no unit tests for `wp_link_pages()`.
	
See #22400.

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git-svn-id: http://core.svn.wordpress.org/trunk@28226 1a063a9b-81f0-0310-95a4-ce76da25c4cd