Remove unused imports and fix some other linter errors.

recipe_engine/unittests/post_process_test.py

 #!/usr/bin/env python
 # Copyright 2016 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.
 
-import copy
-import json
-import os
-import subprocess
 import sys
 import unittest
 
 from collections import OrderedDict
 
 import test_env
 
-import mock
-
 from recipe_engine import post_process
 from recipe_engine import checker
 
 def mkS(name, *fields):
   ret = {
     'name': name,
     'sub_a': ['thing', 'other'],
     'sub_b': 100,
     'sub_c': 'hi',
   }
@@ skipping 108 matching lines @@
 
   def test_dnr_pass_re(self):
     self.expect_fails(0, post_process.DoesNotRunRE, 'x')
 
   def test_dnr_fail_re(self):
     self.expect_fails(3, post_process.DoesNotRunRE, 'b')
 
 
 if __name__ == '__main__':
   sys.exit(unittest.main())