`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.



git-svn-id: https://develop.svn.wordpress.org/trunk@26006 602fd350-edb4-49c9-b593-d223f7449a82