Add tests for KNX light (#58912)

* test lights: simple, brightness, color_temp

* hs, xyy, rgb individual

* test rgb, rgbw

tests/components/knx/conftest.py

@@ -30,6 +30,13 @@ class KNXTestKit:
         # telegrams to an InternalGroupAddress won't be queued here
         self._outgoing_telegrams: asyncio.Queue = asyncio.Queue()
 
+    def assert_state(self, entity_id: str, state: str, **attributes) -> None:
+        """Assert the state of an entity."""
+        test_state = self.hass.states.get(entity_id)
+        assert test_state.state == state
+        for attribute, value in attributes.items():
+            assert test_state.attributes.get(attribute) == value
+
     async def setup_integration(self, config):
         """Create the KNX integration."""