I ran your code, it did pass all the tests from time to time - but mostly it failed one or more of the tests. So you could just try running the tests a bunch and hopefully it will pass once. I made a small change to the tests that made it pass consistently - but in order for that change to get added to your project, you would need to delete you container and start a new one. Learn how here.
Let us know if either of those work. Hopefully the change in the test improves this issue for everyone.