test_rake_clean.rb: skip on Windows

* test/rake/test_rake_clean.rb (create_undeletable_file): skip when
  permission to delete files is different.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43271 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/rake/test_rake_clean.rb

@@ -32,7 +32,13 @@ class TestRakeClean < Rake::TestCase
     FileUtils.touch(file_name)
     FileUtils.chmod(0, file_name)
     FileUtils.chmod(0, dir_name)
-    file_name
+    begin
+      FileUtils.chmod(0644, file_name)
+    rescue
+      file_name
+    else
+      skip "Permission to delete files is different on thie system"
+    end
   end
 
   def remove_undeletable_file