Computing a list from Graphite JSON
You don't want to test the network, the requests or json libraries, you want to check that your method calls requests with a certain parameter (to check if your string interpolation worked and used the correct arguments) and that you return the return value of the JSON method of the response (to check whether you actually used and passed through the object).

At least those are things worthy of testing.  You can simplify your code with mocks if you like them, otherwise I'd say move the class definitions so that you have proper MockRequests and MockResponse classes, maybe the requests with a response constructor argument or so.

And the classes should be as simple as possible, the check for "target" isn't necessary, just check whether the incoming URL has the correct (pre-set) value.  The response shouldn't call anything, it should just return the (again pre-set) JSON object instead.