Missing DEP8 test to cover DNSSEC config

[Bryce Harrington]

The bind9 package has a couple dep8 tests that cover some basics, but what's still needed is testing a dnssec based setup along the lines of the process documented at https://bind9.readthedocs.io/en/latest/dnssec-guide.html, but simplified down.

DNSSEC is "DNS with Security Extensions", allowing digital signatures that allow each DNS response to be verified that the answer did not change during transit (validated), and that the data came from a true source (trusted).

This test will need to short circuit much of the normal process, since we don't want it to communicate across the network, but it should at least verify these two steps are attempted correctly.

Ref:
* https://code.launchpad.net/~lvoytek/ubuntu/+source/bind9/+git/bind9/+merge/440776
* https://bugs.launchpad.net/ubuntu/+source/bind9/+bug/782614