test_require.rb: __FILE__ encoding on Windows

* test/ruby/test_require.rb (assert_require_nonascii_path): OS path
  encoding on Windows is fixed, so encoding of __FILE__ should be it.
  [ruby-core:56498] [Bug #8764]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42523 b2dd03c8-39d4-4d8f-98ff-823fe69b080e