Make `WP_User_Query::prepare_query()` public by allowing it to be passed an...

Make `WP_User_Query::prepare_query()` public by allowing it to be passed an array of args. Previously, if the `WP_User_Query` constructor was not passed args, the object was basically unusable. Adds unit tests, all other tests pass.

Props scribu, for the initial patch.
Fixes #21119.


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git-svn-id: https://core.svn.wordpress.org/trunk@27045 1a063a9b-81f0-0310-95a4-ce76da25c4cd