Todo: add tests around component destruction

[NullVoxPopuli]

Discovered in: #379 (comment)

We found that we could migrate to @ember/destroyable in @glimmer/component's ember-component-manager without failure in ember-source < 3.22 -- this is problematic because @ember/destroyable doesn't exist in older embers -- so it means we are missing important test coverage around destruction.

PRs and help welcome <3