chain: Various minor improvements and unit tests

[sorawit]

Implementation details

• Use HasResult to determine if a request has already been resolved.
• Take reports list in ExecuteEnv creator.
• Add more unit tests for result keeper.
• Refactor handler unit tests and make sure test event emits.

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[render]

Your Render PR Server URL is https://d3n-scan-pr-2146.onrender.com.

Follow its progress at https://dashboard.render.com/static/srv-bs01aco951c4prsmn96g.

[taobun]

So brilliant!