Add unit tests for jth_hess_coord

[amontoison]

close #358

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.40%. Comparing base (f2ad6a4) to head (48a9cf8).
Report is 81 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #399       +/-   ##
===========================================
- Coverage   89.11%   77.40%   -11.71%     
===========================================
  Files           5        8        +3     
  Lines         790     1558      +768     
===========================================
+ Hits          704     1206      +502     
- Misses         86      352      +266     

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