Rework SqlContentEntityStorageSchemaTest

Problem/Motivation

From @longwave in #3351379-14: Only mock methods defined in interfaces → :

I reviewed some of this then realised maybe we are uncovering bugs?

For example in SqlContentEntityStorageSchemaTest the reason we cannot mock EntityTypeManagerInterface is that the constructor calls:

$this->entityType = $entity_type_manager->getActiveDefinition($entity_type->id());

But this method doesn't exist on the interface, so if something swaps in a different EntityTypeManagerInterface there is no guarantee that this method even exists.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet