Add test model for non-linear node (univariate -> multivariate)

[bartvanerp]

Needs to be combined with ReactiveBayes/ReactiveMP.jl#500

[github-actions]

🤖 Code Formatting

Your PR still has some code formatting issues. I've updated PR #506 with the necessary formatting changes.

You can merge that PR into this branch to fix the code style check.

Alternatively, you can run make format locally and push the changes yourself.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.11%. Comparing base (f07bd76) to head (912c9ac).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #505   +/-   ##
=======================================
  Coverage   80.11%   80.11%           
=======================================
  Files          25       25           
  Lines        2012     2012           
=======================================
  Hits         1612     1612           
  Misses        400      400           

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