func-tests: improve zonefile interface

tests-extra/runtests.py

tests-extra/tests/basic/nsec/test.py

tests-extra/tests/basic/nsec3/test.py

tests-extra/tests/ddns/prereq/test.py

tests-extra/tests/records/empty_rdata/test.py

tests-extra/tests/records/nsec_lettercase/test.py

tests-extra/tests/records/obsolete/test.py

tests-extra/tools/dnstest/params.py

tests-extra/tools/dnstest/server.py

tests-extra/tools/dnstest/test.py

tests-extra/tools/dnstest/zonefile.py