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test

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test: 修复 gRPC TestClientMix 数据竞争问题

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en:
When server.start and server.stop are executed concurrently, it is possible to cause concurrent reads and writes to the eventloop field.
The eventloop field is assigned a value before execution is allowed to continue.
Unit Test context: https://github.com/cloudwego/kitex/actions/runs/11010654292/job/30573068741?pr=1559
zh(optional):
当 server.start 与 server.stop 并发执行时，有可能造成 eventloop 字段的并发读写。
eventloop 字段赋值后才允许继续执行。
单测错误上下文：https://github.com/cloudwego/kitex/actions/runs/11010654292/job/30573068741?pr=1559

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