Improve unit test coverage for entity magic methods

Problem/Motivation

Followup from #2854926: Remove unneeded control structures in ContentEntityBase. ContentEntityBase's magic methods have been refactored many, many times over their lifetime, but unit test coverage for their observable behavior has not been added. In particular, breaking __unset() appears to have no explicit or implicit coverage. (__isset() at least appears to have had coverage added: #1868274: Improved EntityNG isset() test coverage.)

Proposed resolution

Add unit test coverage.

Remaining tasks

TBD

User interface changes

N/A

API changes

N/A

Data model changes

N/A