`WP_UnitTestCase::go_to()` tried its best to clean up global space, but ultimately fell short. Because it was blowing away `WP` every time it was called, it was dropping all the query vars that were registered for custom taxonomies and custom post types (ouch).

Introduces `_cleanup_query_vars()`. This is a prerequisite for the unit tests on #20767. All unit tests pass with this change.

See #20767.
Fixes #25818.


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git-svn-id: http://core.svn.wordpress.org/trunk@25937 1a063a9b-81f0-0310-95a4-ce76da25c4cd