add test helpers 3 to peers

[NullVoxPopuli]

This isn't enough to get @ember/test-helpers working.
We need to drop support for node 14 as well, which means, we may as well do a bunch of things that could be breaking, like the v2 conversion, which already drops node 14.

These PRs should be merged/reviewed in order, as they build on each other, so they'll get bigger and bigger unless the prior PRs are merged.

• V2 Addon conversion prep: convert to single-package monorepo. #1060
• Switch to pnpm #1063
• Extract test app #1062
• Convert to native package #1064

my preference would be to merge those above 4, and ignore this PR until the end (due to conflict resolution concerns)

[NullVoxPopuli]

Closing in favor of #1069 (which requires the v2 addon PRs to merged)