dns: Use ldb.SCOPE_SUBTREE in ldap_get_records() routine in tests/dns.py

dns: Use ldb.SCOPE_SUBTREE in ldap_get_records() routine in tests/dns.py

DNS records have the odd property that the DN can be reliably determined by the
name only, so we do not need a subtree search.

However by using a subtree search under the zone we can without
trapping exceptions confirm if the record exists or not in the tests.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=10812

Signed-off-by: Aaron Haslett <aaronhaslett@catalyst.net.nz>
Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Gary Lockyer <gary@catalyst.net.nz>