Fix unifi test_tracked_clients test (#108638)

Fix unifi test_tracked_clients client

This test relied on the sensor platform getting set up and creating
the device before the device_tracker platform was setup since the
device_tracker platform will disable the entity because there is
not matching device entry for it via

https://github.com/home-assistant/core/blob/4ee6735cbbf13c130ee7cf2ca99274141fafc0fc/homeassistant/components/device_tracker/config_entry.py#L336

There is no guarantee the sensor platform will get set up before
the device tracker platform so the test was subject to a race
where it would fail if the device tracker platform was setup first

tests/components/unifi/conftest.py

@@ -121,6 +121,7 @@ def mock_device_registry(hass):
             "00:00:00:00:00:03",
             "00:00:00:00:00:04",
             "00:00:00:00:00:05",
+            "00:00:00:00:00:06",
             "00:00:00:00:01:01",
             "00:00:00:00:02:02",
         )