Addition of puppet8 test

A puppet8 test is now added.

It is marked currently to always allow_failure ignoring the modulesync setting puppet_lastest_allow_failure.

puppet-pson gem is added due to it being needed by teigi at least, see https://gitlab.cern.ch/ai/it-puppet-module-teigi/-/issues/5

Fixes #1 (closed)

Final test run of the real ci-images repository - it-puppet-module-ci_images!123 (closed) as opposed to fake one in here.

Proceeding.