Add tests demonstrated Polymorphic through relationships in M2M admin fields

[bckohan]

closes #375
closes #182

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.09%. Comparing base (ce70799) to head (d90ae47).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #807   +/-   ##
=======================================
  Coverage   84.09%   84.09%           
=======================================
  Files          27       27           
  Lines        1610     1610           
  Branches      253      253           
=======================================
  Hits         1354     1354           
  Misses        195      195           
  Partials       61       61           

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