use 250 messages in cooperative-sticky assignment spec

Author: hoangshopify

spec/integration/cooperative_sticky_assignment_spec.rb

@@ -25,7 +25,7 @@
       { payload: "message-#{n}", partition: n % topic_partitions }
     }
   end
-  let(:message_count) { 20 }
+  let(:message_count) { 250 }
 
   context "during a rebalance" do
     let!(:consumers) { [] }