@ember/test-helpers is trying to import from ember-testing but that is not one of its explicit dependencies

Although the new version is _mostly_ working without this error, it is coming up reliably when the following conditions are met:
* using `@embroider/core@3.4.14` (or probably any other version pre 3.4.15, that hasn't https://github.com/embroider-build/embroider/pull/2075)
* running the build with Embroider and _safe_ mode (optimized is working!)

The root issue seems to be in Embroider, posting this here for visibility.

Reproduce the issue with @embroider/core 3.4.14, Embroider safe mode, and the new @ember/test-helpers version. Compare the explicit dependencies and the import from ember-testing, then review Embroider pull request 2075. Done means the source of the error is confirmed and the relevant project has a clear fix or follow-up.

Written by the indexing model from the issue text.