Tests: field_permissions_entity_field_access field_definition is a base field

Alright, initial test cobbled together from copy/pasting Node and User tests. This is almost certainly buggy -- I haven't run it locally yet -- but I can come back and clean it up or someone else can if I get pulled away for too long.

This may be related to 🐛 500 error with JSONAPI on Base Entity Fields Needs review .