Tests: after using our `grant_super_admin` helper, use `restore_admins` after the assertion to tidy up the `$super_admins` global.

This was otherwise breaking the seemingly unrelated tests added in r9233 when run in multisite. See #5944

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9234 cdf35c40-ae34-48e0-9cc9-0c9da1808c22