How to mock http.Client Do method

Any struct with a method matching the signature you have in your interface will implement the interface. For example, you could create a struct ClientMock

type ClientMock struct {
}

with the method

func (c *ClientMock) Do(req *http.Request) (*http.Response, error) {
    return &http.Response{}, nil
}

You could then inject this ClientMock struct into your GetOverview func. Here’s an example in the Go Playground.