每次测试后的golang测试方法：未定义：testing.M

I am trying to repeat example from golang testing

package main

import (
    "testing"
)

func TestSomeTest(t *testing.T) {}

func TestMain(m *testing.M) { // cleaning after each test}

TestMain

go test

And the compiler says

./testingM_test.go:9: undefined: testing.M

So how to clean after executing every test?