Ensure that when Harness.remove_relation() is called, the relation is…

… treated as-if this was a relation-broken event.

ops/testing.py

@@ -878,8 +878,13 @@ def remove_relation(self, relation_id: int) -> None:
         for unit_name in rel_list_map[relation_id].copy():
             self.remove_relation_unit(relation_id, unit_name)
 
+        prev_broken_id = None  # Silence linter warning.
+        if self._model is not None:
+            prev_broken_id = self._model._relations._broken_relation_id
+            self._model._relations._broken_relation_id = relation_id
         self._emit_relation_broken(relation_name, relation_id, remote_app)
         if self._model is not None:
+            self._model._relations._broken_relation_id = prev_broken_id
             self._model.relations._invalidate(relation_name)
 
         self._backend._relation_app_and_units.pop(relation_id)