ci: do not pass --report-errors to both jobs

We only want to pass --report-errors to the job that allows failures, because otherwise we'd stop publishing the pages as soon as something went wrong.

This is currently the case, because blogs.gnome.org is down, meaning we don't publish anything new.

To make things a bit cleaner, use a common .test-base job as the base, and inherit that for the allow-failure and disallow-failure jobs.