Bugfix/remove attribute ns should not raise not found

I've read ``Contributing.md`` which says to add tests to the end of the test suite but I'm not seeing how I'd be able to do that without breaking the suite's structure. 

I've also modified a couple of tests that were derived from the w3.org test suite. If this is not appropriate, please let me know what I should do. (Looking at the history of the file I did not detect a desire to keep the w3.org tests in their original form. I also did not notice a mechanism for retiring or overriding old tests.)