Add the Collector class to do inverse expectations.

Flush/test/ext/collector.rb

@@ -0,0 +1,18 @@
+class Collector
+  attr_accessor :_messages
+
+  def initialize(*methods)
+    (class << self; self; end).class_eval do
+      methods.each do |m|
+        define_method m do |*args|
+          self._messages << [m, args]
+        end
+      end
+    end
+    self._messages = []
+  end
+
+  def received?(m)
+    self._messages.any? { |method, _| method == m }
+  end
+end