Fix test flake

Fix the following test flake in CI:
```
direct_reply_to_amqp_SUITE > cluster_size_1 > many_replies
    #1. {error,
            {test_case_failed,{timeout,{num_received,2959},{num_missing,41}}}}
```
As documented in https://www.rabbitmq.com/docs/direct-reply-to#caveats-amqp
the volatile queue may drop messages if the writer proc can't send fast
enough.

Author: ansd

deps/rabbit/test/direct_reply_to_amqp_SUITE.erl

@@ -313,7 +313,7 @@ responder_attaches_queue_target(Config) ->
 %% Test that responder can send many messages to requester.
 %% Load test the volatile queue.
 many_replies(Config) ->
-    Num = 3000,
+    Num = 100,
     RequestQueue = atom_to_binary(?FUNCTION_NAME),
     AddrRequestQueue = rabbitmq_amqp_address:queue(RequestQueue),