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/format-erasure_test.go

@@ -104,11 +104,7 @@ func TestFormatErasureEmpty(t *testing.T) {
 // Tests xl format migration.
 func TestFormatErasureMigrate(t *testing.T) {
 	// Get test root.
-	rootPath, err := getTestRoot()
-	if err != nil {
-		t.Fatal(err)
-	}
-	defer os.RemoveAll(rootPath)
+	rootPath := t.TempDir()
 
 	m := &formatErasureV1{}
 	m.Format = formatBackendErasure