Enhance shenyu-plugin-grpc module unit test

[Xiandong Zhong]

#issues https://github.com/dromara/shenyu/issues/1460

Task List:

 Enhance shenyu-plugin-grpc module unit test，make its coverage rate is more 
than 80%

We (@JenkinsChan @babynata @WYUSIG) want to do this task