Clean stray file in addzone test
The !4944 (merged) merge request added a Python test which on Ubuntu 16.04
(Xenial) produces a stray file
./.cache/v/cache/lastfailed. This file
needs to be cleaned for the test not to fail on this OS.
Failing job from the v9.16 backport !4944 (merged) MR: https://gitlab.isc.org/isc-projects/bind9/-/jobs/1666289. (Fails since in daily scheduled pipelines.)