test: fix slice init length in ut

[cuishuang]

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fix

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修复切片初始化长度

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The intention here should be to initialize a slice with a capacity of fRead rather than initializing the length of this slice.

[welkeyever]

Please check the ci: https://github.com/cloudwego/hertz/actions/runs/11047064196/job/30688036659?pr=1205