Add explicit Drush test coverage

Problem/Motivation

While looking into ✨ Misleading message on "admin/config/development/configuration" about configurations handled via config_overlay Active I saw that Config Ignore has explicit Drush test coverage for config import and export. See Drupal\Tests\config_ignore\Functional\ConfigWithTranslationTest. We should do this, as well!

Proposed resolution

Add Drush tests!