Add Matter fixture for Silabs light switch (home-assistant#154701)

Author: lboue

tests/components/matter/conftest.py

@@ -119,6 +119,7 @@ async def integration_fixture(
         "silabs_dishwasher",
         "silabs_evse_charging",
         "silabs_laundrywasher",
+        "silabs_light_switch",
         "silabs_refrigerator",
         "silabs_water_heater",
         "smoke_detector",