[Extern Generation] Add a presubmit script to check externs not being updated

PRESUBMIT_test_mocks.py

Index: PRESUBMIT_test_mocks.py
diff --git a/PRESUBMIT_test_mocks.py b/PRESUBMIT_test_mocks.py
index f68c13441c413d8964258503cac471a01fc3b512..5230c8721073592e17f0479f20da23ac32091f3c 100644
--- a/PRESUBMIT_test_mocks.py
+++ b/PRESUBMIT_test_mocks.py
@@ -109,6 +109,9 @@ class MockFile(object):
   def LocalPath(self):
     return self._local_path
 
+  def AbsoluteLocalPath(self):
+    return self._local_path
+
   def rfind(self, p):
     """os.path.basename is called on MockFile so we need an rfind method."""
     return self._local_path.rfind(p)