pkg/fileutil: wait longer before checking purge results

multiple cpu running may be slower than single cpu running, so it may
take longer time to remove files.
Increase from 5ms to 20ms to give it enough time.

pkg/fileutil/purge_test.go

@@ -86,7 +86,7 @@ func TestPurgeFileHoldingLock(t *testing.T) {
 
 	stop := make(chan struct{})
 	errch := PurgeFile(dir, "test", 3, time.Millisecond, stop)
-	time.Sleep(5 * time.Millisecond)
+	time.Sleep(20 * time.Millisecond)
 
 	fnames, err := ReadDir(dir)
 	if err != nil {
@@ -112,7 +112,7 @@ func TestPurgeFileHoldingLock(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	time.Sleep(5 * time.Millisecond)
+	time.Sleep(20 * time.Millisecond)
 
 	fnames, err = ReadDir(dir)
 	if err != nil {