Remove unused imports and fix some other linter errors.

recipe_engine/unittests/checker_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 json
-import os
-import subprocess
 import sys
 import unittest
 import copy
 
 from collections import OrderedDict
 
 import test_env
 
-import mock
-
 from expect_tests.type_definitions import CheckFrame
 from recipe_engine import checker
 
 
 class TestChecker(unittest.TestCase):
   def sanitize(self, checkframe):
     return checkframe._replace(line=0, fname='')
 
   def mk(self, fname, code, varmap):
     return CheckFrame(
@@ skipping 231 matching lines @@
     a = self.c['a']
     del self.c['a']
     self.c['a'] = a
     self.assertIn(
       "key 'a' is out of order",
       self.v(self.c, self.d))
 
 
 if __name__ == '__main__':
   sys.exit(unittest.main())