Add unit test for bp_activity_get_activity_id()

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8526 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Don't use extract() in bp_activity_get_activity_id(). See #5698

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8527 cdf35c40-ae34-48e0-9cc9-0c9da1808c22