Update dependency @ember/test-helpers to v2.8.1

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
@ember/test-helpers	2.7.0 -> 2.8.1

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Release Notes

emberjs/ember-test-helpers

### [`v2.8.1`](https://togithub.com/emberjs/ember-test-helpers/blob/HEAD/CHANGELOG.md#v281-2022-05-19) [Compare Source](https://togithub.com/emberjs/ember-test-helpers/compare/v2.8.0...v2.8.1) ##### :bug: Bug Fix - [#​1221](https://togithub.com/emberjs/ember-test-helpers/pull/1221) Fix issues with `embroider-optimized` scenarios. ([@​rwjblue](https://togithub.com/rwjblue)) ##### Committers: 1 - Robert Jackson ([@​rwjblue](https://togithub.com/rwjblue)) ### [`v2.8.0`](https://togithub.com/emberjs/ember-test-helpers/blob/HEAD/CHANGELOG.md#v280-2022-05-17) [Compare Source](https://togithub.com/emberjs/ember-test-helpers/compare/v2.7.0...v2.8.0) ##### :rocket: Enhancement - [#​1211](https://togithub.com/emberjs/ember-test-helpers/pull/1211) Implement [embrerjs/rfcs#​785](https://togithub.com/emberjs/rfcs/pull/785) ([@​cafreeman](https://togithub.com/cafreeman)) - Add `rerender` API - Allow `render` to accept components (e.g. ``). ##### :bug: Bug Fix - [#​1216](https://togithub.com/emberjs/ember-test-helpers/pull/1216) Fix usage of `owner.unregister` within `ember-engines` tests ([@​buschtoens](https://togithub.com/buschtoens)) ##### Committers: 2 - Chris Freeman ([@​cafreeman](https://togithub.com/cafreeman)) - Jan Buschtöns ([@​buschtoens](https://togithub.com/buschtoens))

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