Add AssertionFailedError monkeypatch to Test::Unit

features/matchers/match_unless_raises.feature

@@ -8,6 +8,12 @@ Feature: match unless raises
     Given a file named "example.rb" with:
       """
       require 'test/unit/assertions'
+
+      module Test
+        module Unit
+          AssertionFailedError = MiniTest::Assertion if defined?(MiniTest) && !defined?(AssertionFailedError)
+        end
+      end
 
       Spec::Matchers.define :be_the_same_as do |expected|
         extend Test::Unit::Assertions