[V2 Addons] Lint against importing test-support

I'm not sure if linting is the right place to do this, but we have a pretty good standard now on the phrase "test-support", so, in a v2 addon, that would live at:
```
src/
test-support.js
```

and it would not be included in any bundle if not imported (which is great!)
however, I think we could run in to problems if folks accidentally do `import Foo from 'my-addon/test-support'` in app code -- in that the test-support files, qunit-dom, etc would then become part of the app bundle, which we don't want.

ref: https://github.com/embroider-build/embroider/issues/268