Add unit tests for overrides in multi-repo workflow

recipes.py

 #!/usr/bin/env python
 # Copyright 2015 The LUCI Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0
 # that can be found in the LICENSE file.
 
 """Tool to interact with recipe repositories.
 
 This tool operates on the nearest ancestor directory containing an
 infra/config/recipes.cfg.
 """
 
 import argparse
-import collections
 import json
 import logging
 import os
 import shutil
 import subprocess
 import sys
 import tempfile
 
 ROOT_DIR = os.path.dirname(os.path.abspath(__file__))
 sys.path.insert(0, ROOT_DIR)
@@ skipping 250 matching lines @@
 
 class ProjectOverrideAction(argparse.Action):
   def __call__(self, parser, namespace, values, option_string=None):
     p = values.split('=', 2)
     if len(p) != 2:
       raise ValueError("Override must have the form: repo=path")
     project_id, path = p
 
     v = getattr(namespace, self.dest, None)
     if v is None:
-      v = collections.OrderedDict()
+      v = {}
       setattr(namespace, self.dest, v)
 
     if v.get(project_id):
       raise ValueError("An override is already defined for [%s] (%s)" % (
                        project_id, v[project_id]))
     path = os.path.abspath(os.path.expanduser(path))
     if not os.path.isdir(path):
       raise ValueError("Override path [%s] is not a directory" % (path,))
     v[project_id] = path
 
@@ skipping 415 matching lines @@
 
   if not isinstance(ret, int):
     if ret is None:
       ret = 0
     else:
       print >> sys.stderr, ret
       ret = 1
   sys.stdout.flush()
   sys.stderr.flush()
   os._exit(ret)