[NEW TEST] Custom Decorator Testing

**Unit Tests for Custom Decorators**

I created a custom decorator which can be added to Service methods for Caching Data.
But unfortunately cannot find any simple documentation which can provide a way we can test Custom Decorators.
It would be great if you can add something around it.

I found this from the official Github code as a starting point, but since this is such a nice place for all the nest js testing stuff, adding something around decorators would be a plus -https://github.com/nestjs/nest/blob/master/packages/common/test/decorators/cache.decorator.spec.ts