[Improvement] Unit tests support mocking the ShuffleServerNettyHandler class

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)

### Search before asking

- [X] I have searched in the [issues](https://github.com/apache/incubator-uniffle/issues?q=is%3Aissue) and found no similar issues.

### What would you like to be improved?

Unit tests support mocking the ShuffleServerNettyHandler class in order to better cover the business code for the netty part.

We can search for `TODO skip validating for GRPC_NETTY, needs to mock ShuffleServerNettyHandler` in our codes.

### How should we improve?

_No response_

### Are you willing to submit PR?

- [ ] Yes I am willing to submit a PR!