Tests for Windows can run only on Windows

Should not directly require "reline/windows.rb" which should be
loaded by "reline.rb".

test/reline/windows/test_key_event_record.rb

@@ -1,5 +1,5 @@
 require_relative '../helper'
-require 'reline/windows'
+return unless Reline.const_defined?(:Windows)
 
 class Reline::Windows
   class KeyEventRecord::Test < Reline::TestCase