pytest/dns_aging: Correctly check that record is tombstoned

pytest/dns_aging: Correctly check that record is tombstoned

We were passing in as the name parameter to assert_tombstoned() an NDR
Python object, rather than a string. This meant that the LDAP search
filter would look something like this:

(&(objectClass=dnsNode)(name=<dnsp.DnssrvRpcRecord talloc based object at 0x29abee0>))

and we would fail to find any records.

These searches should have a better chance of working if we pass in the
name of the record instead.

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>