+1, I like to test in a similar way. For example if I'm making a CLI, the test will spawn the CLI for each test-case, instead of invoking the code directly. This provides a more realistic flow, and makes it clear which user journeys one is supporting.

Faking responses I often do with environment variables, like:

  ts := httptest.NewServer(...)
  cmd := exec.Command(...)
  cmd.Env = append(os.Environ(), fmt.Sprintf("MYCLI_PROD_ADDR=%s", ts.URL))

Of course, it's better still to go down this turtle stack (e.g. by spawning a local instance of your backend server instead of some faked handlers), but that adds more cost. I find the trade-off OK here.