getncchanges.py: Add a multi-valued linked attribute test

getncchanges.py: Add a multi-valued linked attribute test

Add a test where a source object links to multiple different targets.
First we do the replication without GET_TGT and check that the server
can handle sending a chunk containing only links (in the middle of the
replication). Then we repeat the replication forcing GET_TGT to be used.

To avoid having to create 1500 objects/links, I've lowered the 'max
link sync' setting on the vampire_dc testenv to 250.

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