do not always import tests in setup.py

```
C:\Users\user\Downloads\pydivert-master>python setup.py install
Traceback (most recent call last):
  File "setup.py", line 22, in <module>
    from pydivert.tests import run_test_suites
  File "C:\Users\user\Downloads\pydivert-master\pydivert\tests\__init__.py", line 25, in <module>
    from mock import patch, MagicMock
ImportError: No module named mock
```