Removed reference to MiniTest

We use Test::Unit everywhere, keep it consistent

Author: herwinw

test/test_helper.rb

@@ -32,7 +32,7 @@ def assert_join_threads(threads, message = nil)
           if message
             msg = "#{message}\n#{msg}"
           end
-          raise MiniTest::Assertion, msg
+          raise Test::Unit::AssertionFailedError, msg
         end
         values
       end