Some test tidy for grpc-proxy on ProxyHappySuite

ProxyHappySuite is used to test the behavior of a grpc-proxy server and seems kind of messy. This commit dropped some useless fields and made some logic tidy on SetupSuite and TearDownSuite.

As we have upgraded grpc-go to v1.38.0, we can use the new provided ServerOption grpc.ForceServerCodec to override server codec. ForceServerCodec takes encoding.Codec as parameter and works as a pair of the client side CallOption grpc.ForceCodec. And we can get rid of the deprecated grpc.Codec interface in grpc-proxy.

Refer to the related issue https://github.com/grpc/grpc-go/issues/3694 for details.