FakeClient: Use MagicMock instead of actually channel

This decreases dependencies and prevents problems later on with
solipsism and time_machine.

Signed-off-by: Mathias L. Baumann <[email protected]>

Author: Marenz

src/frequenz/client/dispatch/test/client.py

@@ -4,8 +4,7 @@
 """Fake client for testing."""
 
 from typing import Any, cast
-
-import grpc.aio
+from unittest.mock import MagicMock
 
 from frequenz.client.dispatch import Client
 from frequenz.client.dispatch.test._service import FakeService
@@ -20,7 +19,7 @@ class FakeClient(Client):
 
     def __init__(self) -> None:
         """Initialize the mock client."""
-        super().__init__(grpc.aio.insecure_channel("mock"), "mock")
+        super().__init__(MagicMock(), "mock")
         self._stub = FakeService()  # type: ignore
 
     @property