Enable MSC4115 in complement tests

docker/complement/conf/workers-shared-extra.yaml.j2

@@ -92,8 +92,6 @@ allow_device_name_lookup_over_federation: true
 ## Experimental Features ##
 
 experimental_features:
-  # client-side support for partial state in /send_join responses
-  faster_joins: true
   # Enable support for polls
   msc3381_polls_enabled: true
   # Enable deleting device-specific notification settings stored in account data
@@ -105,6 +103,8 @@ experimental_features:
   # no UIA for x-signing upload for the first time
   msc3967_enabled: true
 
+  msc4115_membership_on_events: true
+
 server_notices:
   system_mxid_localpart: _server
   system_mxid_display_name: "Server Alert"