fix rakefile to run test

Author: khiga8

Rakefile

@@ -5,6 +5,8 @@ require "rake/testtask"
 
 Rake::TestTask.new do |t|
   t.libs << "test"
+  t.libs << "lib"
+  t.test_files = FileList[ENV["TESTS"] || "test/**/*_test.rb"]
 end
 
 desc "Run tests"