Improve go_to() current_blog determining logic in test suite

It's necessary to zero out the current_blog and current_site globals, and then
to use some of the additional fallbacks in ms-settings.php to determine the
correct site.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7818 cdf35c40-ae34-48e0-9cc9-0c9da1808c22