func-tests: add nsec(3) presence check to dnssec tests

tests-extra/tests/dnssec/none_to_nsec/test.py

tests-extra/tests/dnssec/none_to_nsec3/test.py

tests-extra/tests/dnssec/nsec3_to_nsec/test.py

tests-extra/tests/dnssec/nsec3_to_nsec_ddns/test.py

tests-extra/tests/dnssec/nsec_to_nsec3/test.py

tests-extra/tests/dnssec/nsec_to_nsec3_ddns/test.py

tests-extra/tools/dnstest/response.py

tests-extra/tools/dnstest/server.py