Only check if MongoDB is running once durring tests

This should increase the speed of the unit test process because
it changes the check for if MongoDB is running from every time
a test case is run, to only once per test case.

tests/base_case.py

@@ -42,11 +42,12 @@ def tearDown(self):
 
 class ChatBotMongoTestCase(ChatBotTestCase):
 
-    def setUp(self):
+    @classmethod
+    def setUpClass(cls):
         from pymongo.errors import ServerSelectionTimeoutError
         from pymongo import MongoClient
 
-        # Skip these tests if a mongo client is not running
+        # Check if the mongo client is running
         try:
             client = MongoClient(
                 serverSelectionTimeoutMS=0.1
@@ -56,8 +57,6 @@ def setUp(self):
         except ServerSelectionTimeoutError:
             raise SkipTest('Unable to connect to Mongo DB.')
 
-        super(ChatBotMongoTestCase, self).setUp()
-
     def get_kwargs(self):
         kwargs = super(ChatBotMongoTestCase, self).get_kwargs()
         kwargs['database'] = self.random_string()