Test that bans win a join against a race when computing /sync response (#11701)

tests/rest/client/utils.py

@@ -196,6 +196,16 @@ class RestHelper:
             expect_code=expect_code,
         )
 
+    def ban(self, room: str, src: str, targ: str, **kwargs: object):
+        """A convenience helper: `change_membership` with `membership` preset to "ban"."""
+        self.change_membership(
+            room=room,
+            src=src,
+            targ=targ,
+            membership=Membership.BAN,
+            **kwargs,
+        )
+
     def change_membership(
         self,
         room: str,