quorum_queue_SUITE: Use less messages in force_checkpoint_on_queue

[Why]
The default checkpoint interval is 16384. Therefore with 20,000
messages published by the testcase, there is a chance a checkpoint is
created. This would hit an assertion in the testcase which expects no
checkpoints before it forces the creation of one. We see this happening
in CI. Not locally because the testcase runs fast enough.

[How]
The testcase now sends 10,000 messages. This is still a lot of messages
while staying under the default checkpoint interval.

Author: dumbbell

deps/rabbit/test/quorum_queue_SUITE.erl

@@ -1453,7 +1453,7 @@ force_checkpoint_on_queue(Config) ->
     ?assertEqual({'queue.declare_ok', QQ, 0, 0},
                  declare(Ch, QQ, [{<<"x-queue-type">>, longstr, <<"quorum">>}])),
 
-    N = 20_000,
+    N = 10_000,
     rabbit_ct_client_helpers:publish(Ch, QQ, N),
     wait_for_messages_ready([Server0], RaName, N),