Mock容器里面的mock方法还得明确指定targetClass是不是有点不符合语义

https://github.com/alibaba/testable-mock/blob/6616ff1174447a50fdf7db3cd9b8d269f7ca6be0/demo/java-demo/src/test/java/com/alibaba/demo/association/CookerServiceMock.java#L6-L26

是否考虑提供这样一种约定，`targetClass` 为空时就代表Mock容器所关联的被测类中的方法

Start with demo/java-demo/src/test/java/com/alibaba/demo/association/CookerServiceMock.java, especially the referenced mock methods, and trace how the mock container associates with its tested class. Check how an empty targetClass is currently handled. Done means methods without an explicit targetClass resolve against the associated tested class while existing explicit targetClass behavior remains unchanged.

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