Use system zeroconf singleton for homekit (#35502)

Zeroconf instances are expensive so we share a single instance
instead of running multiple.

tests/components/homekit/conftest.py

@@ -29,3 +29,10 @@ def events(hass):
         EVENT_HOMEKIT_CHANGED, ha_callback(lambda e: events.append(e))
     )
     yield events
+
+
+@pytest.fixture
+def mock_zeroconf():
+    """Mock zeroconf."""
+    with patch("homeassistant.components.zeroconf.HaZeroconf") as mock_zc:
+        yield mock_zc.return_value