Extend tests for signed, CNAME-sourced delegations

Extend tests for signed, CNAME-sourced delegations

Extend the "chain" system test with AUTHORITY section checks for signed,
secure delegations.  This complements the checks for signed, insecure
delegations added by commit 26ec4b9a89720e6d5630ad8cf5da6fe357dee32a.

Extend the existing AUTHORITY section checks for signed, insecure
delegations to ensure nonexistence of DS RRsets in such responses.
Adjust comments accordingly.

Ensure dig failures cause the "chain" system test to fail.