Tests: make sure globals are reset when the `go_to` helper method is used to simulate a GET request.

This fixes a problem where `go_to` is used multiple times in one test, and the globals end up with incorrect/unexpected values for the second call; a situation which is likely to occur very infrequently.

Fixes #5965

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9093 cdf35c40-ae34-48e0-9cc9-0c9da1808c22