Add unit tests for runtime/runtime.go and grpc/api.go

[seeflood]

What would you like to be added:
Add unit tests for runtime/runtime.go and grpc/api.go.
These two files are the core engine code of Layotto, responsible for component lifecycle management and grpc request processing respectively.

chinese:
为runtime/runtime.go和grpc/api.go添加单元测试。这两个文件是layotto的核心引擎代码，各自负责组件生命周期管理（启动、注册、初始化）和grpc请求处理，完成此任务可以很好的了解Layotto架构

Why is this needed:
Currently our test coverage is not high :(

[seeflood]

Assigned to @tianjipeng
Thanks for your passion!

[tianjipeng]

OK

[seeflood]

@tianjipeng Hi, are you still working on it?

[tianjipeng]

I wiil commit a pr this weekend

[seeflood]

I wiil commit a pr this weekend

Fine.If you find some code or module difficult to understand,just tell me and I will add comments and document for that module first.