[TEST] ensure @ember-data/serializer package encapsulation

[runspired]

This surfaces an issue that shows we have one more action item to fully encapsulate the @ember-data/serializer package as part of #6166

The issue is that @ember-data/store has an unexpected entanglement with the ember-data package still via the ember-data/-debug util. Since the ember-data package requires the @ember-data/serializer package this makes us not able to drop the package yet.

Action item to move this debug util into its own package.

[runspired]

Realized that only tests and @ember-data/store make use of this so I moved the util into the store package for now. Based on how it uses Model though (which is currently in the Store package`) long term it will either need to become stand-alone or move to the model package