test: add runner cache for better test performance

[LingyuCoder]

Summary

• Add runner cache to prevent reading files multiple times
• Fix slow runtime diff regex
• Use json files as snapshots to prevent building webpack assets which never change

Before:

After:

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

[netlify]

✅ Deploy Preview for rspack canceled.

Name	Link
🔨 Latest commit	8190d9b
🔍 Latest deploy log	https://app.netlify.com/sites/rspack/deploys/67bdbed0fbab6600085e18be

[codspeed-hq]

CodSpeed Performance Report

Merging #9449 will not alter performance

_{Comparing test/runner-cache (8190d9b) with main (ba7556d)}

Summary

✅ 7 untouched benchmarks