bots: Fix unit tests not running in Vagrant.

`test-bots` would not run in Vagrant, displaying
the error "ValueError: no such test method in <class
'bots_test_lib.BotTestCase'>: runTest" This was due to
the `BotTestCase` class inheriting from the TestCase
class, even though it was not a unit test on its own.

This commit removes the inheritance of TestCase and
specifies `test_define` as the `runTest` method in
`TestDefineBot`.

contrib_bots/bots_test_lib.py

@@ -8,14 +8,13 @@ import sys
 import unittest
 
 from mock import MagicMock, patch
-from unittest import TestCase
 
 from run import get_lib_module
 from bot_lib import StateHandler
 from contrib_bots import bot_lib
 from six.moves import zip
 
-class BotTestCase(TestCase):
+class BotTestCase():
 
     def mock_test(self, messages, message_handler, bot_response):
         # type: (List[Dict[str, str]], Function) -> None