Can't run ember test

After many successful times running ember test and ember test -s, I can't run either of them on a specific project. The only thing I can think of that happened since it last ran successfully is that Terminal.app crashed while ember test -s was running.

Now when I run ember test -s on that project, I always get this error:

Global error: Error: Assertion Failed: The initializer 'container-debug-adapter' has 
already been registered at http://localhost:7357/assets/vendor.js, line 38210


Global error: Error: Assertion Failed: The tests file was not loaded. Make sure your 
tests index.html includes "assets/tests.js". at http://localhost:7357/assets/vendor.js, line 38210


Error: Assertion Failed: The initializer 'container-debug-adapter' has already been 
registered at http://localhost:7357/assets/vendor.js, line 38210
Error: Assertion Failed: The tests file was not loaded. Make sure your tests 
index.html includes "assets/tests.js". at http://localhost:7357/assets/vendor.js, line 38210

If I try running ember test -s on a different project, it works fine. So it seems to me that there must be something untracked by git that is sitting somewhere in the project folder that causes this problem.

• Ember: 3.14.1
• Ember CLI: