Fix single test call

Rakefile

@@ -21,7 +21,7 @@ Rake::TestTask.new(:test) do |t|
 
   t.test_files = if file_name
     if test_name
-      ENV["TESTOPTS"] = ENV["TESTOPTS"] + " --name=#{test_name}"
+      ENV["TESTOPTS"] += " --name=test_#{test_name}"
     end
     [file_name]
   else