Unit test for fix 728?

[matsduf]

A bug in Zonemaster-Engine was fixed by PR #728, but no unit tests have been created.

Data was collected with the following commands after #728 (but nothing else) was applied Zonemaster v2019.2. The data files are attached in attach-issue-zonemaster-engine-730.tar.gz
.

zonemaster-cli ed25519.nl --no-ipv6 --raw --test DNSSEC/dnssec13 --save ~/ed25519.nl-dnssec13-with-fix-728.data
zonemaster-cli ip.se --no-ipv6 --raw --test DNSSEC/dnssec13 --save ~/ip.se-dnssec13-with-fix-728.data

The tests files have been verified with the following command, run both with and without #728 applied:

zonemaster-cli ed25519.nl --no-ipv6 --raw --test DNSSEC/dnssec13 --restore  ~/ed25519.nl-dnssec13-with-fix-728.data
zonemaster-cli ip.se --no-ipv6 --raw --test DNSSEC/dnssec13 --restore  ~/ip.se-dnssec13-with-fix-728.data

A test zone that triggers the bug, without the fix, should be set up.