Use +dnssec instead of separate TXT records

Use +dnssec instead of separate TXT records

When testing named instances which are configured to drop outgoing UDP
responses larger than 512 bytes, querying with DO=1 may be used instead
of querying for large TXT records as the effect achieved will be
identical: an unsigned response for a SOA query will be below 512 bytes
in size while a signed response for the same query will be over 512
bytes in size.  Doing this makes all resolution checks in the "legacy"
system test more similar.  Add checks for the TC flag being set in UDP
responses which are expected to be truncated to further make sure that
tested named instances behave as expected.

(cherry picked from commit aaf81ca6ef0a4b3725de9f41efa3f442a6de1c85)