test(model_splitter): cover the horizontal flow barrier package

[jdhughes-dev]

The HFB package had no splitting test, so neither the cellid remapping nor the restriction
that a barrier cannot be split was covered.

• cover the corner four cells share, where two barriers between east west neighbors and two
  between north south neighbors meet in a plus, and check that splitting on either the row or
  the column that junction straddles is an error
• cover barriers on vertical connections, which the splitter never divides because both cells
  of one are in the same stack
• check that each model keeps its own barriers, that none are lost, and that the heads
  reconstruct to the original model

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.4%. Comparing base (556c088) to head (e7cc42d).
⚠️ Report is 211 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2824      +/-   ##
===========================================
+ Coverage     55.5%    73.4%   +17.9%     
===========================================
  Files          644      659      +15     
  Lines       124135   132188    +8053     
===========================================
+ Hits         68947    97096   +28149     
+ Misses       55188    35092   -20096     

see 585 files with indirect coverage changes

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