test_tempfile.rb: fix test on Windows

* test/test_tempfile.rb (test_tempfile_finalizer_does_not_run_if_unlinked):
  open file cannot be deleted on Windows.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43155 b2dd03c8-39d4-4d8f-98ff-823fe69b080e