Improve fieldable entity test/example class

Problem/Motivation

As part of the ✨ Support entities that are neither content nor config entities Needs work fix a simple/dummy _just_ fieldable entity type was defined for the sake of test coverage. However, if we would like to take interface separation seriously we should have a better test coverage in Drupal core with a better test entity that warrants codes in core do not depend on "hidden" features.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet