Add test for asdf installation

Check that asdf and its plugins install correctly.

• No .tool-versions change, the job is not running https://gitlab.com/gitlab-org/gitlab-docs/-/pipelines/797869636
• Change in .tool-versions:
  • The job is running and failing with the Ruby version not found https://gitlab.com/gitlab-org/gitlab-docs/-/jobs/3883998098
  • The job is running and succeeding https://gitlab.com/gitlab-org/gitlab-docs/-/jobs/3883812447