Fake more things in MockIMAP4

This fixes test failures which crept in.

Author: Menno Finlay-Smits

imapclient/testable_imapclient.py

@@ -30,6 +30,8 @@ def __init__(self, *args: Any, **kwargs: Any):
         self.use_uid = True
         self.sent = b""  # Accumulates what was given to send()
         self.tagged_commands: Dict[Any, Any] = {}
+        self.untagged_responses: Dict[Any, Any] = {}
+        self.capabilities = []
         self._starttls_done = False
 
     def send(self, data: bytes) -> None: