selftest: Add test for deleted single-valued link conflict

selftest: Add test for deleted single-valued link conflict

Currently we're only testing the case where the links have been modified
independently on 2 different DCs and both the links are active. We also
want to test the case where one link is active and the other is deleted.

Technically, this isn't really a conflict - the links involve different
target DNs, and the end result is still only one active link.

It's still probably worth having these tests to prove that fixing bug
13055 doesn't break anything.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13055

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