Upgrade ember-test-helpers and node

[MelSumner]

If merged, this PR updates ember-test-helpers to 3.0.2 (including peer dep declaration).

Details:

• removes ember-lts-3.28 from ember-try scenario
• fixes a typo in addon-test-support/adapter.js
• adds @ember/string v3.1.1
• updates @ember/test-helpers to v3.0.3
• updates ember-source to >=4.0.0
• drops node 14
• updates node and yarn versions for volta
• removes unused imports in types/tests.ts
• updates types/tests.ts to use async/await instead of RenderingTestContext (deprecated)

[NullVoxPopuli]

I'd like to merge these PRs first https://github.com/emberjs/ember-qunit/pulls/@me
all the PR descriptions can give context, and the needed breaking changes are throughout the PRs.

[gilest]

I'd like to merge these PRs first

This link might work better for people who aren't you 😅 https://github.com/emberjs/ember-qunit/pulls/NullVoxPopuli

Unless I'm mistaken this will be blocked until Node 14 support is dropped

[NullVoxPopuli]

Whoops, thanks!

[MelSumner]

Right, so, my plan is to do two releases:

1. drop old node, upgrade ember-test-helpers version, release new major
2. merge in the PRs from @NullVoxPopuli then do a new major after that

This is BECAUSE of all the issues we faced with ember-test-helpers, it feels a little more pragmatic to do it this way. BUT I am running into issues just trying to do this first one so we'll see. If I can't resolve it soon I'll consider a different approach.

[NullVoxPopuli]

looks like @ember/string needs to be added to the test app

[NullVoxPopuli]

just making notes for release,
we're dropping support for:

• @ember/test-helpers < 3.0.3
• ember-source < 4
• node < 16
• this.element -- folks should use qunit.dom or [data-test-whatever] and find
• this.render -- await render is easy find and replace

[MelSumner]

for this.render, folks would need to remember to async function (assert) + await render

[NullVoxPopuli]

Yup, something to call out in the changelog