Add tests for bad mutation parents

[benjeffery]

Fixes #3310

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.80%. Comparing base (8e6b1e6) to head (3440f0f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3321   +/-   ##
=======================================
  Coverage   89.79%   89.80%           
=======================================
  Files          29       29           
  Lines       31056    31062    +6     
  Branches     5685     5687    +2     
=======================================
+ Hits        27888    27894    +6     
  Misses       1779     1779           
  Partials     1389     1389           

Flag	Coverage Δ
c-tests	86.84% <ø> (ø)
lwt-tests	80.38% <ø> (ø)
python-c-tests	87.05% <ø> (ø)
python-tests	98.84% <ø> (+<0.01%)	⬆️
python-tests-no-jit	33.55% <ø> (-0.03%)	⬇️
python-tests-numpy1	50.10% <ø> (-0.04%)	⬇️

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see 3 files with indirect coverage changes

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[petrelharp]

LGTM!