Fix group filtering for the 'popular' type.

See r7087.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7111 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Adds tests for 'type=popular' BP_Groups_Group::get()

see r7087, r7111

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7112 cdf35c40-ae34-48e0-9cc9-0c9da1808c22