In test_avatars_on_non_root_blog(), use go_to_root() to establish root blog

This ensures that all globals have been reset by the time avatar directories
are queried, preventing test failures when run in certain configs

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7191 cdf35c40-ae34-48e0-9cc9-0c9da1808c22