pubsub: Use distinct client IDs for test subscriptions. (#7178)

Fixes #7176. Some of the benchmarks create a bunch of different subscriptions all sharing the same query. These were all using the same client ID, which violates one of the subscriber rules. Ensure each subscriber gets a unique ID.

This has been broken as long as this library has been in the repo—I tracked it back to bb9aa85d and it was already failing there, so I think this never really worked. I'm not sure these test anything useful, but at least now they run.

libs/pubsub/pubsub_test.go

@@ -518,7 +518,8 @@ func benchmarkNClientsOneQuery(n int, b *testing.B) {
 	ctx := context.Background()
 	q := query.MustParse("abci.Account.Owner = 'Ivan' AND abci.Invoices.Number = 1")
 	for i := 0; i < n; i++ {
-		subscription, err := s.Subscribe(ctx, clientID, q)
+		id := fmt.Sprintf("clientID-%d", i+1)
+		subscription, err := s.Subscribe(ctx, id, q)
 		if err != nil {
 			b.Fatal(err)
 		}