Fix intermittently deadlocking #post_off test

Author: pitr-ch

spec/concurrent/agent_spec.rb

@@ -20,10 +20,9 @@ module Concurrent
     end
 
     context '#send_off' do
-      subject { Agent.new 2 }
+      subject { Agent.new 2, executor: executor }
 
       it 'executes post and post-off in order' do
-        pending 'may cause deadlock'
         subject.post { |v| v + 2 }
         subject.post_off { |v| v * 3 }
         subject.await