adds TestAgent for test and development environments

This PR adds a `TestAgent`, which implements all the same methods as the original `Agent` (now renamed to `StandardAgent`, leaving `Agent` open for the interface). 

With this addition, you can conditionally set a variable of type `Agent` to either a `TestAgent` or `StandardAgent` (depending on your env, or whatever you like) and without changing anything in your code, you can avoid reporting of development/test metrics to NewRelic.