Add a regression test for polymorphic iteration

[bckohan]

• regenerate migrations, fix test
• remove prints
• move test to regressions

PR #254 is broken, maybe because it is so old, I am superceding it with this PR.

Normally I wouldn't accept PRs like this but the user claims a regression and more is more with testing.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.71%. Comparing base (9399f7d) to head (2d6f9f0).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #671   +/-   ##
=======================================
  Coverage   74.71%   74.71%           
=======================================
  Files          21       21           
  Lines        1337     1337           
  Branches      211      211           
=======================================
  Hits          999      999           
  Misses        261      261           
  Partials       77       77           

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