[Bug] Memory Leak in RouterService + rendering tests

[hmajoros]

🐞 Describe the Bug

In any rendering test which uses the router on ember 3.24, ember's RouterService is retaining an instance of the EmberRouter class, and is holding on to one instance per test module. in large apps, this can cause the browser to crash while running the entire test suite due to browser OOM.

This change seems to have been introduced in this PR. The RouterService caches the EmberRouter and never cleans up the reference.

🔬 Minimal Reproduction

• Running an app using ember 3.24+, create a basic rendering test which uses the router service.
• Using Chrome's Memory tab, run the test and capture a heap dump
• Inspect the heap dump and notice instance of PrivateRouter class

😕 Actual Behavior

RouterService is hanging on to a reference to EmberRouter.

🤔 Expected Behavior

References to EmberRouter should be cleaned up in between tests.

🌍 Environment

• Ember: 3.24.4 or higher
• Node.js/npm: v14.15.1
• OS: Mac
• Browser: Chrome

➕ Additional Context

N/A

[hmajoros]

Proposed fix would be adding a willDestroy hook in the RouterService that essentially just does:

willDestroy() {
  super.willDestroy(...arguments);
  this[ROUTER] = null;
}

I tested this fix locally against my codebase and it resolved the memory leak.