Polyfill memory leak prevention.

[rwjblue]

This polyfills the changes from qunitjs/qunit#1279 to older versions of QUnit.

---

Prior to the changes in this PR, all module and test callbacks are retained (forever). This may not seem significant, but as folks use closure scope to store data across tests (which is very common).

For example, prior to the changes in this PR the following would retain the local variable:

QUnit.module('top', function(hooks) {
  let largeThing;

  hooks.beforeEach(function() {
    largeThing = new LargeThing();
  });

  hooks.afterEach(function() {
    largeThing.destroy();
  });

  test('something that uses largeThing', function(assert) {
    // ...snip...
    largeThing.someMethod();
  });
});

In larger test suites, this really adds up...

[Turbo87]

assuming you've tested this on a real world project, this seems good to me 👍

[rwjblue]

assuming you've tested this on a real world project

Confirm, I've tested in a few different apps and also confirmed that prior known leaks (like the one in the description) are resolved.