Adds some @group groupings to groups tests

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7293 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Tell BP_Group_Member_Query queries to order by FIELD()

BP_Group_Member_Query works by first querying for a list of group members
that match the query params, sorted by date_modified. Then, it feeds those ids
to the BP_User_Query parent class. In order to maintain the proper
date_modified sort, we have to override BP_User_Query's ORDER BY param, and
replace it with ORDER BY FIELD(), using the user ids from the first query.

This fixes a regression in BP 1.8, where the default sort order for
bp_group_has_members() was inadvertantly changed to user_id DESC, rather than
the legacy date_modified that existed before the introduction of
BP_Group_Member_Query.

Fixes #5106

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7294 cdf35c40-ae34-48e0-9cc9-0c9da1808c22