docs: add local testing section to helion-hackathon.md

[yf225]

Summary

• Adds a "Testing Locally" section to docs/helion-hackathon.md documenting the eval.py local testing workflow from reference-kernels#123
• Covers test, benchmark, both, and profile modes with example commands
• Placed before "Writing a Helion Submission" for natural reading order

Test plan

• Verified markdown renders correctly
• Commands match the eval.py interface in reference-kernels

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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