De-flake MSC3030 backfill test with Synapse worker replication

Spawning from matrix-org/synapse#14028 (comment)

The reason they failed before was because we're fighting against
stale caches across workers racing while waiting for invalidation,
see matrix-org/synapse#13185 (comment)

> Here is what happens:
>
>  1. `serverB` has `event1` stored as an `outlier` from previous requests (specifically from MSC3030 jump to date pulling in a missing `prev_event` after backfilling)
>  1. Client on `serverB` calls `/messages?dir=b`
>  1. `serverB:client_reader1` accepts the request and drives things
>  1. `serverB:client_reader1` has some backward extremities in range and requests `/backfill` from `serverA`
>  1. `serverB:client_reader1` processes the events from backfill including `event1` and puts them in the `_event_persist_queue`
>  1. `serverB:master` picks up the events from the `_event_persist_queue` and persists them to the database, de-outliers `event1` and invalidates its own cache and sends them over replication
>  1. `serverB:client_reader1` starts assembling the `/messages` response and gets `event1` out of the stale cache still as an `outlier`
>  1. `serverB:client_reader1` responds to the `/messages` request without `event1` because `outliers` are filtered out
>  1. `serverB:client_reader1` finally gets the replication data and invalidates its own cache for `event1` (too late, we already got the events from the stale cache and responded)

Author: MadLittleMods

tests/msc3030_test.go

@@ -203,6 +203,16 @@ func TestJumpToDateEndpoint(t *testing.T) {
 				contextResResBody := client.ParseJSON(t, contextRes)
 				paginationToken := client.GetJSONFieldStr(t, contextResResBody, "end")
 
+				// Hit `/messages` until `eventA` has been backfilled and replicated across
+				// workers.
+				fetchUntilMessagesResponseHas(t, remoteCharlie, roomID, func(ev gjson.Result) bool {
+					if ev.Get("event_id").Str == eventA.EventID {
+						return true
+					}
+
+					return false
+				})
+
 				// Paginate backwards from eventB
 				messagesRes := remoteCharlie.MustDoFunc(t, "GET", []string{"_matrix", "client", "r0", "rooms", roomID, "messages"}, client.WithContentType("application/json"), client.WithQueries(url.Values{
 					"dir":   []string{"b"},