getncchanges.py: Add some GET_TGT test cases

getncchanges.py: Add some GET_TGT test cases

test_repl_get_tgt:
- Adds 2 sets of objects
- Links one set to the other
- Changes the order so the target object comes last in the
  replication (which means the client has to use GET_TGT)
- Checks that when GET_TGT is used that we have received all target
  objects we need to resolve the linked attibutes
- Checks that we expect to receive the linked attributes *before*
  the last chunk is sent (by default, Samba sends all the links at
  the end, so this fails)
- Checks that we eventually receive all expected objects, and all
  links we receive match what is expected

test_repl_get_tgt_chain:
  This adds the linked attributes in a more complicated chain. We add
  300 objects, but the links for 100 objects will point to a linked
  chain of 200 objects.
  This was mainly to determine whether or not Windows follows the
  target object (i.e. whether it sends all the links for the target
  object as well). It turns out Windows maintains its own linked
  attribute DB, so it sends the links based on USN.

Note that the 2 testenvs fail for different reasons. promoted_dc fails
because it is sending all the linked attributes last. vampire_dc fails
because it doesn't support GET_TGT yet, so it sends the link before the
peer knows about the target object.

Note that to test against vampire_dc (rather than the ad_dc_ntvfs DC),
we need to send the GetNCChanges requests to DC2 instead of DC1.
I've left the DC numbering scheme as is, but I've addeed a test_ldb_dc
handle to drs_base.py - it defaults to DC1, but tests can override it
easily and still have everything work.

While running the new tests through autobuild, I noticed an intermittent
LDAP_ENTRY_ALREADY_EXISTS failure in the test setup(). This appears to
be due to a timing issue in the background replication between the
multiple testenvs. Adding some randomness so that the test base OU is
unique seems to avoid the problem.

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