Support custom code coverage checks in CI

[jelhan]

I noticed that some adopted addons, which are not yet migrated to GitHub Actions, added code coverage checks to their TravisCI pipeline. Would be great if we could support this use case out of the box.

• ember-stripe-elements: https://github.com/adopted-ember-addons/ember-stripe-elements/blob/7c5c8ef681a2d507293d8dc4661fd0d4d2c50da7/.travis.yml#L49-L50
• ember-light-table: https://github.com/adopted-ember-addons/ember-light-table/blob/69b25a6333e5784d4ce56dd3c4dcc30cc3d145fa/.travis.yml#L53-L54
• ember-file-upload: https://github.com/adopted-ember-addons/ember-file-upload/blob/556075f97c2ceada0ac11c3bcfbd734b84bd6245/.travis.yml#L42-L43