Test against embroider

[NullVoxPopuli]

Tests against strictest embroider -- which we weren't doing before.

This is important because in the v2 addon conversion, I ran in to

• instanceof fails in classic mode embroider-build/ember-auto-import#588
• Let webpack handle test vs app layering embroider-build/ember-auto-import#503

which is actually good, because test-waiters needs to be resilient to:

• copies of itself being loaded
• copies of other libraries using the same waiter names

Adding embroider to the matrix helps verify that the problem is with auto-import having duplicate test-time dependencies, vs embroider fixing the problem and having only one copy of a dependency.

Why not use the try matrix, like in the blueprint?

• try matrix already has/had embroider specified, but none of the test-apps were set up with maybeEmbroider -- it turns out as well that the ember-concurrency stuff isn't compatible (it's an old version, and I'm considering a separate PR to move all the thennable and generator tests into the main test app, but without using ember-concurrency, so that it's better declared what we're actually supporting, and more unit-testy, rather than e2e-y)
• because debugging is easier this way -- ember-try messes with the local node_modules which makes getting back to a non-ember-try state a bit harder (sometimes pnpm i isn't enough).
• because we want to be able to debug both classic and embroider quickly, it makes sense that someone would even have both running at the same time.