Beef up test coverage

[hcho3]

• Test with categorical data
• Test multi-target models
• Test with randomly generated cases

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 81.80%. Comparing base (69a8169) to head (377b1b8).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #23 +/- ## ========================================== - Coverage 82.95% 81.80% -1.16% ========================================== Files 64 64 Lines 2529 2506 -23 ========================================== - Hits 2098 2050 -48 - Misses 431 456 +25 ```

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