Cast/Sonos: create config entry if manually configured (#15630)

* Cast/Sonos: create config entry if manually configured

* Add test for helper

homeassistant/helpers/config_entry_flow.py

@@ -72,6 +72,18 @@ class DiscoveryFlowHandler(data_entry_flow.FlowHandler):
 
         return await self.async_step_confirm()
 
+    async def async_step_import(self, _):
+        """Handle a flow initialized by import."""
+        if self._async_in_progress() or self._async_current_entries():
+            return self.async_abort(
+                reason='single_instance_allowed'
+            )
+
+        return self.async_create_entry(
+            title=self._title,
+            data={},
+        )
+
     @callback
     def _async_current_entries(self):
         """Return current entries."""