Complement `TestRestrictedRoomsRemoteJoinInMSC3787Room/Join_should_fail_when_left_allowed_room` flake

This issue has been migrated from [#15637](https://github.com/matrix-org/synapse/issues/15637).

---

E.g. https://github.com/matrix-org/synapse/actions/runs/5026762073/jobs/9015490796 (`complement (monolith, Postgres)`)

```
TestRestrictedRoomsRemoteJoinInMSC3787Room/Join_should_fail_when_left_allowed_room (5.25s)
restricted_rooms_test.go:131: @alice:hs1 MustSyncUntil: timed out after 5.143191793s. Seen 6 /sync responses. Checkers:
[t=86.496749ms] Response #1: SyncTimelineHas(!hkmMVjuRCITcvzGBnO:hs1): check function did not pass while iterating over 8 elements: [{"type":"m.room.create","sender":"@alice:hs1","content":{"room_version":"10","creator":"@alice:hs1"},"state_key":"","origin_server_ts":1684518613639,"unsigned":{"age":1666},"event_id":"$qwKY43_YgBFV1WEkBlPNUdzZliqV1hyoyP3YyHmZCOE"},{"type":"m.room.member","sender":"@alice:hs1","content":{"membership":"join","displayname":"Alice"},"state_key":"@alice:hs1","origin_server_ts":1684518613716,"unsigned":{"age":1589},"event_id":"$LDeOT-uuAkjcyXuvnehtwExN7o4jS0-nAGmtudRFSbs"},{"type":"m.room.power_levels","sender":"@alice:hs1","content":{"users":{"@alice:hs1":100},"users_default":0,"events":{"m.room.name":50,"m.room.power_levels":100,"m.room.history_visibility":100,"m.room.canonical_alias":50,"m.room.avatar":50,"m.room.tombstone":100,"m.room.server_acl":100,"m.room.encryption":100},"events_default":0,"state_default":50,"ban":50,"kick":50,"redact":50,"invite":50,"historical":100}
,"state_key":"","origin_server_ts":1684518613745,"unsigned":{"age":1560},"event_id":"$9O8RVzBEN4PDLx8Z4jAc_pxis9lNDEQ0yySZa9qaijk"},{"type":"m.room.join_rules","sender":"@alice:hs1","content":{"join_rule":"public"},"state_key":"","origin_server_ts":1684518613753,"unsigned":{"age":1552},"event_id":"$r7JZwi_K42S0GApfOsOEqU_7i4R1uHfAFpJdS4HzxrY"},{"type":"m.room.history_visibility","sender":"@alice:hs1","content":{"history_visibility":"shared"},"state_key":"","origin_server_ts":1684518613754,"unsigned":{"age":1551},"event_id":"$2gbZrWsS0R1W_0SEBIuPNH8JlO1Kj4J5Ib31HHRzn3Y"},{"type":"m.room.name","sender":"@alice:hs1","content":{"name":"Allowed Room"},"state_key":"","origin_server_ts":1684518613754,"unsigned":{"age":1551},"event_id":"$joDCHA_LSjTd8BwiR8LQPH_zKv0Tk2eKGytX_wI1qSA"},{"content":{"avatar_url":null,"displayname":"Bob","membership":"join"},"origin_server_ts":1684518614363,"sender":"@bob:hs2","state_key":"@bob:hs2","type":"m.room.member","unsigned":{"age":6},"event_id":"$ejIUzOSzjRY2_xIQPgh6rtNiDFkeHMSI0j
HcCzeIbRg"},{"content":{"displayname":"Bobby","membership":"join"},"origin_server_ts":1684518614620,"sender":"@bob:hs2","state_key":"@bob:hs2","type":"m.room.member","unsigned":{"replaces_state":"$ejIUzOSzjRY2_xIQPgh6rtNiDFkeHMSI0jHcCzeIbRg","prev_content":{"avatar_url":null,"displayname":"Bob","membership":"join"},"prev_sender":"@bob:hs2"},"event_id":"$lBTIJW9-avu-YI8iyX_anLpBodGNRB6XDd6UdWYi-_8"}]
[t=1.101307491s] Response #2: SyncTimelineHas(!hkmMVjuRCITcvzGBnO:hs1): Key rooms.join.!hkmMVjuRCITcvzGBnO:hs1.timeline.events does not exist
[t=2.112043139s] Response #3: SyncTimelineHas(!hkmMVjuRCITcvzGBnO:hs1): Key rooms.join.!hkmMVjuRCITcvzGBnO:hs1.timeline.events does not exist
[t=3.12223981s] Response #4: SyncTimelineHas(!hkmMVjuRCITcvzGBnO:hs1): Key rooms.join.!hkmMVjuRCITcvzGBnO:hs1.timeline.events does not exist
[t=4.132285594s] Response #5: SyncTimelineHas(!hkmMVjuRCITcvzGBnO:hs1): Key rooms.join.!hkmMVjuRCITcvzGBnO:hs1.timeline.events does not exist
[t=5.143189693s] Response #6: SyncTimelineHas(!hkmMVjuRCITcvzGBnO:hs1): Key rooms.join.!hkmMVjuRCITcvzGBnO:hs1.timeline.events does not exist,
```