In bp_has_members(), only auto-set user_id if looking at a user friends page

Previously, the 'user_id' param was being pre-filled anytime you were looking
at a user page. As a result, it was impossible to do certain kinds of query
logic while looking at friends pages.

Adds unit tests for the change.

Fixes #4938

git-svn-id: https://buddypress.svn.wordpress.org/trunk@6941 cdf35c40-ae34-48e0-9cc9-0c9da1808c22