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 5d5b221760c0dab765dd10645ca088f2daf24681..aa9f77da12f0265fce30477aa6eadb472ae23952 100755
--- a/bin/cros_au_test_harness.py
+++ b/bin/cros_au_test_harness.py
@@ -358,12 +358,12 @@ 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(target_image_path)
+    self.PrepareBase(base_image_path)
     self.UpdateImage(target_image_path)
     self.VerifyImage(100)