When `ember-auto-import` is present, avoid `app.import` of `QUnit`
itself. Without this change, users (when they have `peerDependencies`
setup) would get an error like:

```
Uncaught Error: QUnit has already been defined.
```

This is because we were previously always `app.import`ing `QUnit`
itself, but then `ember-auto-import` sees:
`import { test, module } from 'qunit';` in a test file alongside the
`package.json` having an entry for `qunit` so it attempts to bundle it
**again**.

This is the path _away_ from custom build/app.import shenanigans and
helps us down the path of avoiding specialness in an Embroider focused
world.

The `vendor/ember-cli/test-support-suffix.js` file overrides a file
built into ember-cli's build pipeline and prevents this built-in
`Testem.hookIntoTestFramework` invocation:

https://github.com/ember-cli/ember-cli/blob/v3.20.0/lib/broccoli/test-support-suffix.js#L3-L5

The reason this override is needed, is because the built-in ember-cli
invocation of `Testem.hookIntoTestFramework()` happens _during_
`test-support.js` evaluation, which (when we are leveraging Embroider or
ember-auto-import) QUnit is not present (since its not eagerly
evaluated, we only are guaranteed it is present when we import it). As
such the `Testem.hookIntoTestFramework` invocation has been moved into
our `addon-test-support/index.js` which is only ran once QUnit is
actually defined/available.

Now that the addon itself depends on ember-auto-import, it no longer
makes sense to test as an ember-try scenario.