selftest: Add some tests for linked attribute conflicts

selftest: Add some tests for linked attribute conflicts

Currently we have tests that check we can resolve object conflicts, but
these don't test anything related to conflicting linked attributes.
This patch adds some basic tests that checks that Samba can resolve
conflicting linked attributes.

This highlights some problems with Samba, as the following tests
currently fail:
- test_conflict_single_valued_link: Samba currently can't resolve a
  conflicting targets for a single-valued linked attribute - the
  replication exits with an error.
- test_link_deletion_conflict: If 2 DCs add the same linked attribute,
  currently when they resolve this conflict the RMD_VERSION for the
  linked attribute incorrectly gets incremented. This means the version
  numbers get out of step and subsequent changes to the linked attribute
  can be dropped/ignored.
- test_full_sync_link_conflict: fails for the same reason as above.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>
Autobuild-User(master): Garming Sam <garming@samba.org>
Autobuild-Date(master): Mon Sep 18 09:56:41 CEST 2017 on sn-devel-144