test: use T.TempDir to create temporary test directory (#15400)

This commit replaces `ioutil.TempDir` with `t.TempDir` in tests. The
directory created by `t.TempDir` is automatically removed when the test
and all its subtests complete.

Prior to this commit, temporary directory created using `ioutil.TempDir`
needs to be removed manually by calling `os.RemoveAll`, which is omitted
in some tests. The error handling boilerplate e.g.
	defer func() {
		if err := os.RemoveAll(dir); err != nil {
			t.Fatal(err)
		}
	}
is also tedious, but `t.TempDir` handles this for us nicely.

Reference: https://pkg.go.dev/testing#T.TempDir

Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>

cmd/os-readdir_test.go

@@ -67,25 +67,16 @@ type result struct {
 	entries []string
 }
 
-func mustSetupDir(t *testing.T) string {
-	// Create unique test directory.
-	dir, err := ioutil.TempDir(globalTestTmpDir, "minio-list-dir")
-	if err != nil {
-		t.Fatalf("Unable to setup directory, %s", err)
-	}
-	return dir
-}
-
 // Test to read empty directory.
 func setupTestReadDirEmpty(t *testing.T) (testResults []result) {
 	// Add empty entry slice for this test directory.
-	testResults = append(testResults, result{mustSetupDir(t), []string{}})
+	testResults = append(testResults, result{t.TempDir(), []string{}})
 	return testResults
 }
 
 // Test to read non-empty directory with only files.
 func setupTestReadDirFiles(t *testing.T) (testResults []result) {
-	dir := mustSetupDir(t)
+	dir := t.TempDir()
 	entries := []string{}
 	for i := 0; i < 10; i++ {
 		name := fmt.Sprintf("file-%d", i)
@@ -107,7 +98,7 @@ func setupTestReadDirFiles(t *testing.T) (testResults []result) {
 
 // Test to read non-empty directory with directories and files.
 func setupTestReadDirGeneric(t *testing.T) (testResults []result) {
-	dir := mustSetupDir(t)
+	dir := t.TempDir()
 	if err := os.MkdirAll(filepath.Join(dir, "mydir"), 0o777); err != nil {
 		t.Fatalf("Unable to create prefix directory \"mydir\", %s", err)
 	}
@@ -134,7 +125,7 @@ func setupTestReadDirSymlink(t *testing.T) (testResults []result) {
 		t.Skip("symlinks not available on windows")
 		return nil
 	}
-	dir := mustSetupDir(t)
+	dir := t.TempDir()
 	entries := []string{}
 	for i := 0; i < 10; i++ {
 		name1 := fmt.Sprintf("file-%d", i)
@@ -241,7 +232,7 @@ func TestReadDirN(t *testing.T) {
 	}
 
 	for i, testCase := range testCases {
-		dir := mustSetupDir(t)
+		dir := t.TempDir()
 
 		for c := 1; c <= testCase.numFiles; c++ {
 			err := ioutil.WriteFile(filepath.Join(dir, fmt.Sprintf("%d", c)), []byte{}, os.ModePerm)