Upgrade ember-qunit to the v5 style of tests, using the highest suppo…

[NullVoxPopuli]

Work extracted from: #1573
which is all part of an effort to get C.I. green again without changing the support matrix.

I'd tried to go for a smaller, more focused PR: #1575
But,

• it seems there is too much inter-connecteness between the dependencies for any smaller PR alone to fix CI
• it's not really worth it to split up PRs for "changelog generation", when the whole PR is "internal" and doesn't cause any external-noticeable differences in behavior.

This PR

• uses the highest versions of addons/dependencies that will still support ember-cli-typescript's minimum ember version: 3.28
• adds auto-import and webpack as devDependecies, as those are required for the dummy app (due to ember-qunit@v5)
• re-rolls the lockfile
• addresses types issue due to floating dependencies