getncchanges.py: Add test for GET_ANC and linked attributes

getncchanges.py: Add test for GET_ANC and linked attributes

Add a basic test that when we use GET_ANC and the parents have linked
attributes, then we receive all the expected links and all the expected
objects by the end of the test.

This extends the test code to track what linked attributes get received
and check whether they match what's present on the DC.

Also made some minor cleanups to store the received objects/links each
time we successfully receive a GETNCChanges response (this saves the
test case having to repeat this code every time).

Note that although this test involves linked attributes, it shouldn't
exercise the GET_TGT case at all.

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