Add mock queue implementation

spec/support/mock_queue.rb

@@ -0,0 +1,32 @@
+class MockQueue
+  attr_reader :messages
+
+  def initialize
+    @messages = []
+    @subscription_id = 0
+    @subscriptions = {}
+  end
+
+  def subscribe(&block)
+    @subscription_id += 1
+    @subscriptions[@subscription_id] = block
+    @subscription_id
+  end
+
+  def push(message)
+    @messages.push(message)
+    @subscriptions.values.each do |callback|
+      callback.call(message)
+    end
+    message
+  end
+
+  def clear_messages!
+    @messages = []
+  end
+
+  def unsubscribe(subscription_id)
+    @subscriptions.delete(subscription_id)
+    nil
+  end
+end