Add homekit_controller tests (#20457)

* Add a test for a homekit_controller switch

* Add a test for a homekit_controller lightbulb

* Add a test for homekit_controller thermostat

* Changes from review

* Patch utcnow to known time in HK tests

* Neater fixture use per review

tests/components/homekit_controller/conftest.py

@@ -0,0 +1,14 @@
+"""HomeKit controller session fixtures."""
+import datetime
+from unittest import mock
+
+import pytest
+
+
+@pytest.fixture
+def utcnow(request):
+    """Freeze time at a known point."""
+    start_dt = datetime.datetime(2019, 1, 1, 0, 0, 0)
+    with mock.patch('homeassistant.util.dt.utcnow') as dt_utcnow:
+        dt_utcnow.return_value = start_dt
+        yield dt_utcnow