// production
var println = fmt.Println

func MyFuncThatPrints() {
    println("some message")
}

// test
func TestMyFuncThatPrints() {
    expectedLog := "some message"

    // replace the package level function with your own
    println = func (v ...[]interface{}) {
        // assert here
    }
}

// prod file
type printer func(v ...interface{})

func InfoWithFunction(printFn printer, v ...interface{}) {
    var level []interface{}
    level = append(level, "INFO")
    data := append(level, v...)
    printFn(data...)
}

// test file
func TestInfoWithFunction(t *testing.T) {
    expected := "INFO hello world\n"
    logString := "hello world"

    // mock a function
    printFn := func(v ...interface{}) {
        result := fmt.Sprintln(v...)
        assert.Equal(t, expected, result)
    }
    // and pass it to the function under test
    InfoWithFunction(printFn, logString)
}