Move /t_ms delta class parsing to separate file and add tests

tests/conftest.py

@@ -8,7 +8,14 @@ from fbchat import Message, Mention
 
 @pytest.fixture(scope="session")
 def session():
-    return object()  # TODO: Add a mocked session
+    class FakeSession:
+        # TODO: Add a further mocked session
+        user_id = "31415926536"
+
+        def __repr__(self):
+            return "<FakeSession>"
+
+    return FakeSession()
 
 
 @pytest.fixture(scope="session")