Test fns not special forms

[moquist]

Add tests for steps 6 and 7 so atom-related, cons, and concat are implemented as fns and not special forms.

Pull request requirements:

• Commits are well written and well organized.
• Commits for a specific implementation should be prefixed with
  the implementation name.
• Github Actions CI passes all checks (including self-host)

GHA CI had 2 failures, but these failures also occur with the upstream [178b7a8] commit:
https://github.com/moquist/mal/actions/runs/14948899199/job/41995831023#step:9:122
https://github.com/moquist/mal/actions/runs/14948899199/job/41995831107#step:10:5

The same failures, with my changes:
https://github.com/moquist/mal/actions/runs/14947777877/job/41993343841#step:9:122
https://github.com/moquist/mal/actions/runs/14947777877/job/41993343887#step:10:5

[kanaka]

Sorry for the slow response. I haven't been watching my notifications very closely. Looks like there is also a failure in xslt directly due to the new tests. If you want to tackle a really challenging implementation, you could try and fix xslt. Otherwise, I would be fine with moving these to optional tests, noting that xslt fails on them, and then filing an issue about it.