Check `BP_User_Query` when using `friends_check_friendship_status()`.

Previously, `friends_check_friendship_status()` did not reference the data
already queried in the members loop via `BP_User_Query`.  As a result, if
a user was logged in and viewing the member directory, this added twenty
additional DB queries to the page.

This commit:
* Adds an additional property - `friendship_status` - to `bp_friends_filter_user_query_populate_extras()`. This property references the various friendship statuses from `BP_Friends_Friendship::check_is_friend()`
* Checks this new property in `friends_check_friendship_status()` before querying the DB
* Adds unit tests

Props r-a-y, boonebgorges.

Fixes #5703.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8514 cdf35c40-ae34-48e0-9cc9-0c9da1808c22