fix: Fix split test event logging. (#32299)

In a refactor, this call to runtime.publish didn't
actually match the API properly, causing runtime errors
on LMS.

xmodule/split_test_block.py

@@ -409,7 +409,7 @@ class SplitTestBlock(  # lint-amnesty, pylint: disable=abstract-method
             )
             raise
         else:
-            self.runtime.publish('xblock.split_test.child_render', {'child_id': child_id})
+            self.runtime.publish(self, 'xblock.split_test.child_render', {'child_id': child_id})
             return Response()
 
     def get_icon_class(self):