Tests: in `test_avatars_on_non_root_blog`, don't use `go_to` to change the WP site globals.

In my local testing, I've removed the code entirely. If we find it fails in edge cases later,
we should reinstate this using `switch_to_blog`.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9275 cdf35c40-ae34-48e0-9cc9-0c9da1808c22