Add REST test coverage for Layout Builder

Problem/Motivation

Once #2942975: [PP-1] Expose Layout Builder data to REST and JSON:API lands, we should ensure that REST operations on Layout Builder fields behaves in an expected manner.

Proposed resolution

Write REST test coverage for Layout Builder. It makes sense to do this for entity overrides, but defaults may need to be tested as well, since modules like JSON API expose config entities via REST by default.

Remaining tasks

Discuss scope of test coverage, write a patch.

User interface changes

None.

API changes

None.

Data model changes

None.