getncchanges.py: Add test for GET_ANC and GET_TGT combined

getncchanges.py: Add test for GET_ANC and GET_TGT combined

The code has to handle needing GET_ANC and GET_TGT in combination, i.e.
where we fetch the target object for the linked attribute and the target
object's parent is unknown as well. This patch adds a test case to
exercise this code path.

The second part of this test exercises GET_ANC/GET_TGT for an
incremental replication, where the objects are getting filtered by an
uptodateness-vector/HWM.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>