Tests hang

Here is a demo that hangs consistently in Firefox in headless mode, or being run via `ember test --server`

https://github.com/amk221/-ember-waiters-issue/commit/4f41d022212a8f3949ce5510ed3a0e2a68f29d0a#diff-539124910b2d9910320f2b3c5516639c1a1e417847d0ae2769c1f839e13b713dR20-R31

I am secretly hoping its not an issue specific to Firefox, but just shows up in it easier. And that if fixed would iron out flakes I see in our test suite of > 8000, where occasionally we'll see pending test waiters cause the tests to eventually bomb out even in Chrome