[MNT] `testing` remaining cleanup

[MatthewMiddlehurst]

Cleans up some utility and data loading files from testing. Leaves make_series as that is being dealt with in #2310.

After this and #2310, the only remaining thing to do for the testing module is review the current checks.

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• [ ] Run pre-commit checks for all files
• [ ] Run mypy typecheck tests
• [ ] Run all pytest tests and configurations
• [ ] Run all notebook example tests
• [ ] Run numba-disabled codecov tests
• [ ] Stop automatic pre-commit fixes (always disabled for drafts)
• [ ] Disable numba cache loading
• [ ] Push an empty commit to re-run CI checks