test_command_processor.rb: fix for mswin/mingw and test for directory

* test/shell/test_command_processor.rb (test_system_external): fix for
  mswin and mingw.  one of EXECUTABLE_EXTS is needed but shell.rb does
  not support it.

* test/shell/test_command_processor.rb (test_system_directory): test
  not to find directory.  [ruby-core:57235] [Bug #8918]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43041 b2dd03c8-39d4-4d8f-98ff-823fe69b080e