README

You need to run tests with:

go test -tags=test

a.go

// +build !test

package a

func Hello() string {
	return "hello"
}

a_test.go

package a

import "testing"

func TestHello(t *testing.T) {
	if got, expected := Hello(), "hello from tests"; got != expected {
		t.Errorf("got = %q; expected = %q", got, expected)
	}
}

test.go

// +build test

package a

func Hello() string {
	return "hello from tests"
}