patman: Drop import of test_util in test_util

This module doesn't need to import itself. It causes problems on
very old Python 3 (e.g. 3.4.0). Drop it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/patman/test_util.py b/tools/patman/test_util.py
index 4d28d9f..aac58fb 100644
--- a/tools/patman/test_util.py
+++ b/tools/patman/test_util.py
@@ -11,7 +11,6 @@
 import unittest
 
 from patman import command
-from patman import test_util
 
 from io import StringIO