main.go

package main

import (
	"fmt"
	"log"
	"os"
	"path/filepath"
)

func Run(d string) error {
	{
		f, err := os.Create(filepath.Join(d, "1.txt"))
		if err != nil {
			return err
		}
		fmt.Fprintln(f, "one")
	}
	{
		f, err := os.Create(filepath.Join(d, "2.txt"))
		if err != nil {
			return err
		}
		fmt.Fprintln(f, "two")
	}
	{
		f, err := os.Create(filepath.Join(d, "3.txt"))
		if err != nil {
			return err
		}
		fmt.Fprintln(f, "three")
	}
	return nil
}

func main() {
	if err := Run(os.Getenv("TARGET")); err != nil {
		log.Fatalf("!! %+v", err)
	}
}

main_test.go

package main

import (
	"io/ioutil"
	"testing"
)

func TestRun(t *testing.T) {
	d := t.TempDir()
	t.Logf("outdir: %s", d)
	if err := Run(d); err != nil {
		t.Errorf("!! %+v", err)
	}

	files, err := ioutil.ReadDir(d)
	if err != nil {
		t.Errorf("!!! %+v", err)
	}

	wantFiles := []string{"1.txt", "2.txt", "3.txt"}
	if len(files) != len(wantFiles) {
		t.Errorf("mismatch the number of files, got=%d, want=%d", len(files), len(wantFiles))
	}

	for i, f := range files {
		if f.Name() != wantFiles[i] {
			t.Errorf("files[%d], %s != %s", i, f.Name(), wantFiles[i])
		}
	}
}

x.log

# test log
getenv TMPDIR
open /var/folders/b7/2rk7xp2d0hb2r21zbzjwxb_m0000gn/T/TestRun822825880/001/1.txt
open /var/folders/b7/2rk7xp2d0hb2r21zbzjwxb_m0000gn/T/TestRun822825880/001/2.txt
open /var/folders/b7/2rk7xp2d0hb2r21zbzjwxb_m0000gn/T/TestRun822825880/001/3.txt
open /var/folders/b7/2rk7xp2d0hb2r21zbzjwxb_m0000gn/T/TestRun822825880/001
stat /var/folders/b7/2rk7xp2d0hb2r21zbzjwxb_m0000gn/T/TestRun822825880/001/1.txt
stat /var/folders/b7/2rk7xp2d0hb2r21zbzjwxb_m0000gn/T/TestRun822825880/001/2.txt
stat /var/folders/b7/2rk7xp2d0hb2r21zbzjwxb_m0000gn/T/TestRun822825880/001/3.txt
open /var/folders/b7/2rk7xp2d0hb2r21zbzjwxb_m0000gn/T