bless_test_results: Add a dry-run option.

[jgfouca]

This is helpful for testing btr mechanics without changing baselines

Checklist

• My code follows the style guidelines of this project (black formatting)
• I have performed a self-review of my own code
• My changes generate no new warnings
• I have added tests that exercise my feature/fix and existing tests continue to pass
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding additions and changes to the documentation