Add simple update test to pfq.

bin/cros_au_test_harness.py

Index: bin/cros_au_test_harness.py
diff --git a/bin/cros_au_test_harness.py b/bin/cros_au_test_harness.py
index 5a0182c4815812a99af29a0655e34456917b87f8..da4e537b6a84f7d054a3bf1664f9d4b1d961dec8 100755
--- a/bin/cros_au_test_harness.py
+++ b/bin/cros_au_test_harness.py
@@ -372,13 +372,13 @@ class AUTest(object):
     self.AttemptUpdateWithFilter(DelayedFilter())
 
   def SimpleTest(self):
-    """A simple update  that updates the target image to itself.
+    """A simple update that updates once from a base image to a target.
 
     We explicitly don't use test prefix so that isn't run by default.  Can be
     run using test_prefix option.
     """
-    self.PrepareBase(self.target_image_path)
-    self._UpdateImage(self.target_image_path)
+    self.PrepareBase(self.base_image_path)
+    self.PerformUpdate(self.target_image_path, self.base_image_path)
     self.VerifyImage(100)