Plex tests cleanup and additions (#37117)

tests/components/plex/helpers.py

@@ -0,0 +1,9 @@
+"""Helper methods for Plex tests."""
+from homeassistant.components.plex.const import DOMAIN, WEBSOCKETS
+
+
+def trigger_plex_update(hass, plex_server):
+    """Call the websocket callback method."""
+    server_id = plex_server.machineIdentifier
+    websocket = hass.data[DOMAIN][WEBSOCKETS][server_id]
+    websocket.callback()