Update tests to use Julia v1.11

[odow]

Closes #127

[odow]

By luck, the change from Julia 1.10 to 1.11 now means that features contained no points x <= [0.5, 0.3], so we never trained a tree to predict 3.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.78%. Comparing base (d8e5b0e) to head (22c461b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #133      +/-   ##
==========================================
- Coverage   90.13%   89.78%   -0.36%     
==========================================
  Files          21       21              
  Lines         517      509       -8     
==========================================
- Hits          466      457       -9     
- Misses         51       52       +1     

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