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/test-utils_test.go

@@ -1452,11 +1452,6 @@ func getListenNotificationURL(endPoint, bucketName string, prefixes, suffixes, e
 	return makeTestTargetURL(endPoint, bucketName, "", queryValue)
 }
 
-// returns temp root directory. `
-func getTestRoot() (string, error) {
-	return ioutil.TempDir(globalTestTmpDir, "api-")
-}
-
 // getRandomDisks - Creates a slice of N random disks, each of the form - minio-XXX
 func getRandomDisks(N int) ([]string, error) {
 	var erasureDisks []string