Run unit tests on OpenBSD in GitLab CI

Run unit tests on OpenBSD in GitLab CI

Unlike other maintained BIND branches, the "main" BIND branch does not
require Kyua for running unit tests, which has been an obstacle for
adding an OpenBSD unit test job to GitLab CI.  Experiments show that a
complete BIND unit test suite completes in a few minutes on OpenBSD and
that unit tests are not as severely affected by OpenBSD performance
issues as system tests are.  Add a GitLab CI job which runs unit tests
on OpenBSD to every pipeline.