chore(deps): update dependency @ember/test-helpers to v2.2.8

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This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
@ember/test-helpers	2.2.6 -> 2.2.8

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Release Notes

emberjs/ember-test-helpers

### [`v2.2.8`](https://togithub.com/emberjs/ember-test-helpers/blob/master/CHANGELOG.md#v228-2021-06-15) [Compare Source](https://togithub.com/emberjs/ember-test-helpers/compare/v2.2.7...v2.2.8) ##### :bug: Bug Fix - [#​1050](https://togithub.com/emberjs/ember-test-helpers/pull/1050) Remove unneeded `treeForVendor` ([@​rwjblue](https://togithub.com/rwjblue)) ##### Committers: 1 - Robert Jackson ([@​rwjblue](https://togithub.com/rwjblue)) ### [`v2.2.7`](https://togithub.com/emberjs/ember-test-helpers/blob/master/CHANGELOG.md#v227-2021-06-15) [Compare Source](https://togithub.com/emberjs/ember-test-helpers/compare/v2.2.6...v2.2.7) ##### :bug: Bug Fix - [#​1047](https://togithub.com/emberjs/ember-test-helpers/pull/1047) Ensure jQuery.ajax pending request detection works with Ember 3.27 under Embroider ([@​stefanpenner](https://togithub.com/stefanpenner)) ##### :house: Internal - [#​1035](https://togithub.com/emberjs/ember-test-helpers/pull/1035) removed unused import ([@​helgablazhkun](https://togithub.com/helgablazhkun)) ##### Committers: 3 - Olga Torkhanova ([@​helgablazhkun](https://togithub.com/helgablazhkun)) - Stefan Penner ([@​stefanpenner](https://togithub.com/stefanpenner)) - [@​dependabot-preview\[bot\]](https://togithub.com/apps/dependabot-preview)

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