Thinking

func TestGetReleaseTagMessage(t *testing.T) {
        cases := []struct {
                f           FakeReleaseInfoer
                repo        string
                expectedMsg string
                expectedErr error
        }{
                {
                        f: FakeReleaseInfoer{
                                Tag: "v0.1.0",
                                Err: nil,
                        },
                        repo:        "doesnt/matter",
                        expectedMsg: "The latest release is v0.1.0",
                        expectedErr: nil,
                },
                {
                        f: FakeReleaseInfoer{
                                Tag: "v0.1.0",
                                Err: errors.New("TCP timeout"),
                        },
                        repo:        "doesnt/foo",
                        expectedMsg: "",
                        expectedErr: errors.New("Error querying GitHub API: TCP timeout"),
                },
        }

        for _, c := range cases {
                msg, err := getReleaseTagMessage(c.f, c.repo)
                if !reflect.DeepEqual(err, c.expectedErr) {
                        t.Errorf("Expected err to be %q but it was %q", c.expectedErr, err)
                }

                if c.expectedMsg != msg {
                        t.Errorf("Expected %q but got %q", c.expectedMsg, msg)
                }
        }
}