Refactoring tests, add Bossan::Test::TestCase.

There are some tests in test/ directory, however all tests include below part:

    begin
      $stderr.puts RUBY_DESCRIPTION
      pid = fork do
        trap(:INT) { Bossan.stop }
        Bossan.listen(DEFAULT_HOST, DEFAULT_PORT)
        Bossan.run(App.new)
      end
    
      Process.detach pid
    
      unless server_is_wake_up?
        $stderr.puts "bossan won't wake up until you love it ..."
        exit 1
      end
    
      err = MiniTest::Unit.new.run(ARGV)
      exit false if err && err != 0
    
    ensure
      Process.kill(:INT, pid)
    end

This patch refactors tests, it moves server start/stop codes to test/bossan_test.rb
We can use Bossan::Test::TestCase instead of Test::Unit::TestCase. It automatically starts/stops server each tests.

And, add test endpoint test/driver.rb, so `rake test` simply kick it.