test: make CopyPlugin.test faster and more stable

[xc2]

Summary

related: #6084

failing CI samples:

• https://github.com/web-infra-dev/rspack/actions/runs/8505294188/job/23293849274#step:9:65
• https://github.com/web-infra-dev/rspack/actions/runs/8507670127/job/23300468307#step:8:66

packages/rspack test: FAIL tests/copyPlugin/CopyPlugin.test.js (66.335 s, 34 MB heap size)
packages/rspack test:   ● CopyPlugin › watch mode › should include all files when multiple patterns used
packages/rspack test:     TypeError: Cannot read properties of undefined (reading 'compilation')
packages/rspack test:        9 | 	const assets = {};
packages/rspack test:       10 |
packages/rspack test:     > 11 | 	Reflect.ownKeys(stats.compilation.assets)
packages/rspack test:          | 	                      ^
packages/rspack test:       12 | 		.filter(a => a !== "main.js")
packages/rspack test:       13 | 		.forEach(asset => {
packages/rspack test:       14 | 			assets[transformWindowPath(asset)] = readAsset(asset, compiler, stats);
packages/rspack test:       at readAssets (tests/copyPlugin/helpers/readAssets.js:11:24)
packages/rspack test:       at tests/copyPlugin/helpers/run.js:191:25
packages/rspack test:       at shutdown (dist/Watching.js:99:21)
packages/rspack test:       at Watching.finalCallback [as _done] (dist/Watching.js:116:13)
packages/rspack test:       at onCompile (dist/Watching.js:2[61](https://github.com/web-infra-dev/rspack/actions/runs/8505294188/job/23293849274#step:9:62):18)
packages/rspack test:       at dist/Compiler.js:424:28
packages/rspack test:       at Hook.eval [as callAsync] (eval at create (../../node_modules/tapable/lib/HookCodeFactory.js:33:10), <anonymous>:6:1)
packages/rspack test:       at Hook.CALL_ASYNC_DELEGATE [as _callAsync] (../../node_modules/tapable/lib/Hook.js:18:14)
packages/rspack test:       at dist/Compiler.js:420:41
packages/rspack test:       at dist/Compiler.js:747:[65](https://github.com/web-infra-dev/rspack/actions/runs/8505294188/job/23293849274#step:9:66)

Checklist

• Tests updated (or not required). - no need
• Documentation updated (or not required). - no need

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[LingyuCoder]

LGTM, Thanks