Container firewall test

[troglobit]

Description

• Minor cleanups
• test: basic firewall container #598
• Proposal for how a test section/group could look

Checklist

Tick relevant boxes, this PR is-a or has-a:

• Bugfix
  • Regression tests
  • ChangeLog updates (for next release)
• Feature
  • YANG model change => revision updated?
  • Regression tests added?
  • ChangeLog updates (for next release)
  • Documentation added?
• Test changes
  • Checked in changed Readme.adoc (make test-spec)
  • Added new test to group Readme.adoc and yaml file
• Code style update (formatting, renaming)
• Refactoring (please detail in commit messages)
• Build related changes
• Documentation content changes
  • ChangeLog updated (for major changes)
• Other (please describe):

[troglobit]

Ping @jovatn, this PR also contains a proposal for how a test section/group could look. It's a litmus test to see if we're aligned or if we need to discuss this a bit more. (Re: discussion you, me, and @axkar had the other day.)