s4:torture/ldap/basic.c - add a basic test for referral return

s4:torture/ldap/basic.c - add a basic test for referral return

I implemented this referral test in C since the LDB python API isn't capable
to extract referrals from search result sets (there the result sets are simple
lists which contain only the matching entries).

First I enhanced the RootDSE test to return all partition base DNs in a new
null-terminated list "partitions". Then I used this in my referrals test which
I've implemented in the LDB api since I needed some certain DN functions.