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/define/test_define.py

@@ -42,7 +42,7 @@ class TestDefineBot(TestCase):
         messages.append(message2)
         return messages
 
-    def test_define(self):
+    def runTest(self):
         # type: None -> None
         # Edit bot_module to test different bots, the below code can be looped for all the bots.
         bot_module = os.path.join(our_dir, "define.py")