Add integration tests for bp_group_has_membership_requests()

These tests check to see that the results of a default query are correct, and
that the results are formatted as expected. This is in anticipation of
refactoring related to #5440

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8065 cdf35c40-ae34-48e0-9cc9-0c9da1808c22