Stupid github doesn't let me do slashes in filenames, so os-mock.go is os/mock.go and os-stdlib.go is os/stdlib.go.

foo_test.go

// All you need to do is: `go test -tags mock .` to have the mocked version.
package main

import (
	"fmt"
	"testing"

	"github.com/tiborvass/foo/os"
)

func TestFoo(t *testing.T) {
	fmt.Println(os.Getenv("SHELL"))
}

os-mock.go

// +build mock

package os

func Getenv(key string) string {
	return key
}

os-stdlib.go

// +build !mock

package os

import "os"

func Getenv(key string) string {
	return os.Getenv(key)
}