The test code should fail with the current, unfixed code, due to the bug described here.  You will need to:

1. figure out which conditions will trigger the wrong behavior
2. set up a scenario where you can detect whether the behavior is correct
3. code this as a new test method in test_minidom.py
4. make the fix and ensure that the new test passes, as well as all other tests

After you've done this, you'll have something ready to be a PR. It will likely still require some work, e.g. cleaning up the code, adding/improving comments, conforming to our code style guidelines, and adding a NEWS entry.  However, since this will be one of your first PRs, I suggest first just posting your test and fix as a PR.

Note that creating a PR will also trigger the test suite to be automatically run on several platforms; after ~15 minutes take a look at the results, and if there are any failures you should address them ASAP.