Introduces a 'meta_query' parameter for the bp_has_groups() stack

This will allow plugin and theme authors to customize their group directories
based on data stored in the groupmeta table, using a powerful syntax familiar
from WP_Query.

Also adds unit tests for BP_Groups_Group::get() for the meta_query parameter,
as well as an integration test for the entire bp_has_groups() chain of
functions.

Fixes #3521

git-svn-id: https://buddypress.svn.wordpress.org/trunk@6950 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
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git-svn-id: https://buddypress.svn.wordpress.org/trunk@6951 cdf35c40-ae34-48e0-9cc9-0c9da1808c22