Add unit test for collate groups deprecation warning

Including the groups key in the feature config was hitting a bug, so we should probably add a unit test to ensure this still works when only specifying `entity_id` (but raises a deprecation warning), even though we're moving away from supporting specifying this key generally.

No file or existing test is named. Start by locating the feature-config handling and current tests for the groups and entity_id keys; reproduce the configuration using only entity_id and confirm the deprecation warning is emitted while the behavior still works. Done means the regression is covered by a unit test.

Written by the indexing model from the issue text.