Update CHANGELOG.md for new testing framework

[liamcervante]

We have documentation in draft, and I will write an upgrade guide but they won't be available for the alpha. Not sure if I should mention that somewhere in the changelog?

[github-actions]

Reminder for the merging maintainer: if this is a user-visible change, please update the changelog on the appropriate release branch.

[apparentlymart]

I think this changelog entry is appropriate for the current stage of development. We can always expand it later if we have more to say.

For an alpha release we'd typically include all of the relevant information in the "request for testing" community forum topic, so that the forum topic becomes the hub for everything needed to participate in the alpha feedback process. That typically works well because we can more easily update and add to a forum topic on the fly, whereas updating the changelog and documentation has a heavier process cost.

[github-actions]

I'm going to lock this pull request because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active contributions.
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