Run unit tests on OpenBSD in CI

When OpenBSD was introduced to GitLab CI in !2434 (merged), unit test job was missing because OpenBSD "ships with an old version of kyua which does not handle skipped tests properly", noting "[t]hese jobs will be added when we move away from using kyua in the future as the test code itself works fine".

Given that main does not require Kyua, we should introduce unit tests on OpenBSD in the CI.

While working on !2699 (merged) I was able to build kyua 0.13 by modifying the Ports tree. Although I wasn't 100% successful having a clean update to the Ports component, copying resulting kyua binary to /usr/local/bin/ and copying /usr/ports/pobj/kyua-0.13/fake-amd64/usr/local/share/kyua/ to /usr/local/share/ did the trick.

I think we should hack our OpenBSD image to have newer kyua so we can enable unit tests (and work with the Ports community to fix this for good).