pytest: samba-tool dns: allow valid updates

pytest: samba-tool dns: allow valid updates

Without this patch we will get errors like this when in-place RPC
updates start to work:

   AssertionError: unexpectedly None : Successfully updated record
   '192.168.0.1' to '192.168.0.1', even though the latter is of type
   'A' where 'A' was expected.

That's because we have always rejected updates that try to modify an
existing record. We shouldn't.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>