how unit test files automatically run

test unit autorun tests.rb

    ##
    # Registers MiniTest::Unit to run tests at process exit

    def self.autorun
      at_exit {
        next if $! # don't run if there was an exception
        exit_code = MiniTest::Unit.new.run(ARGV)
        exit false if exit_code && exit_code != 0
      } unless @@installed_at_exit
      @@installed_at_exit = true
    end