tests: add String#md5 method

tests/test.d/util.rb

@@ -1,3 +1,5 @@
+require 'digest/md5'
+
 $message_mutex = Mutex.new
 def show_message message
   $message_mutex.lock
@@ -35,6 +37,12 @@ class Array
   end
 end
 
+class String
+  def md5
+    Digest::MD5.hexdigest self
+  end
+end
+
 # install md5 handler
 case RUBY_PLATFORM
 when /darwin/