[go-nuts] Restrict http.Client to only localhost for testing

As recommended, I share my http.Clients throughout my program. As I'm
setting up unit testing, I'm realizing that there are a number of places
where I'm making external API requests that don't necessarily need to be
tested. Is there a way to overwrite my client in testing with a custom
transport or roundtripper that restricts the fetches to only specific
allowed urls? I only want to allow localhost + connections to httptest
servers that I spin up for mocks.

To be clear, I know how to set up my tests so I can overwrite the
destination url or pass in interfaces. This is more of a failsafe for
mistakes or jr devs, to keep from accidentally creating thousands of
unnecessary clients and campaigns if someone forgets to set things up
properly. :)

Thanks,
Derek