getncchanges.py: Add a test for dropped cross-partition links

getncchanges.py: Add a test for dropped cross-partition links

Samba would drop linked attributes that span partitions if it didn't
know about the target object. This patch adds a test that exposes the
problem.

I've re-used the code from the previous re-animation test to do this.
I've also added a very basic DcConnection helper class that basically
stores the connection state information the drs_base.py uses for
replication. This allows us to switch the DC we want to replicate from
easily. This approach could potentially be retro-fitted to some of the
existing test cases, as it allows us to test both the DRS client code
and server code at the same time.

Note this test case relates to the code change for commit
fae5df891c11f642cb.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
BUG: https://bugzilla.samba.org/show_bug.cgi?id=12972
Reviewed-by: Garming Sam <garming@catalyst.net.nz>