Regexp.escape(RUBY_PLATFORM)

* test/ruby/test_require.rb (TestRequire#test_require_nonascii_path):
  RUBY_PLATFORM should escape as Regexp,
  because RUBY_PLATFORM may contain '.'.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40173 b2dd03c8-39d4-4d8f-98ff-823fe69b080e