Return 0 as return_code explicitly.

This is a follow-up of http://go/r8g/5080. Even when all tests pass, the
desktop notification said tests failed, since there is no return_code at
the end of Main().

Change-Id: I60a6c2fc3f583531787f954032884ad45be59a22
diff --git a/tools/test.py b/tools/test.py
index 0cd8296..9774207 100755
--- a/tools/test.py
+++ b/tools/test.py
@@ -149,6 +149,8 @@
         archive_failures()
       return return_code
 
+  return 0
+
 if __name__ == '__main__':
   return_code = Main()
   if return_code != 0:
@@ -156,3 +158,4 @@
   else:
     notify.notify("Tests passed.")
   sys.exit(return_code)
+