test: use T.TempDir to create temporary test directory

The directory created by `T.TempDir` is automatically removed when the
test and all its subtests complete.

Reference: https://pkg.go.dev/testing#T.TempDir
Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>

weed/util/chunk_cache/chunk_cache_on_disk_test.go

@@ -4,14 +4,11 @@ import (
 	"bytes"
 	"fmt"
 	"math/rand"
-	"os"
 	"testing"
 )
 
 func TestOnDisk(t *testing.T) {
-
-	tmpDir, _ := os.MkdirTemp("", "c")
-	defer os.RemoveAll(tmpDir)
+	tmpDir := t.TempDir()
 
 	totalDiskSizeInKB := int64(32)