Fixed tests on Windows (I hope) by forcing platform for
  platform-dependent tests.

  Fixed File.exists? warnings.

  Improved testing infrastructure.

* test/rubygems:  ditto.

* test/rdoc/test_rdoc_rubygems_hook.rb:  Switch to util_spec like
  RubyGems.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43651 b2dd03c8-39d4-4d8f-98ff-823fe69b080e