For checks querying a named instance with "dnssec-accept-expired yes;"
set, authoritative responses have a TTL of 300 seconds.  Assuming empty
resolver cache, TTLs of RRsets in the ANSWER section of the first
response to a given query will always match their authoritative
counterparts.  Also note that for a DNSSEC-validating named resolver,
validated RRsets replace any existing non-validated RRsets with the same
owner name and type, e.g. cached from responses received while resolving
CD=1 queries.  Since TTL capping happens before a validated RRset is
inserted into the cache and RRSIG expiry time does not impose an upper
TTL bound when "dnssec-accept-expired yes;" is set and, as pointed out
above, the original TTLs of the relevant RRsets equal 300 seconds, the
RRsets in the ANSWER section of the responses to expiring.example/SOA
and expired.example/SOA queries sent with CD=0 should always be exactly
120 seconds, never a lower value.  Make the relevant TTL checks stricter
to reflect that.

(cherry picked from commit a85cc414)