Fixed #23289 -- Added mock as a test dependency.

This will raise an `AttributeError` on Python 3.3+ since `unittest.mock` is a submodule:

```py
$ python3.4
Python 3.4.1 (default, Jul 11 2014, 12:38:25) 
[GCC 4.6.3] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import unittest
>>> unittest.mock
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'module' object has no attribute 'mock'
>>> import unittest.mock
>>> 
```