Configurable fields that are created only for update path tests should be defined with custom_storage = TRUE

Problem/Motivation

In a few update path tests, we create fixtures for configurable field storages so we can test an update hook that touches their settings.

However, those fixtures do not go through the regular "field install" process (no database tables, no last installed schema entries) because they're not needed for the purpose of the update test.

This is not a problem at the moment, but it will become one once we start using the last installed definitions for entity type and field storage CRUD operations in #2976035: Entity type CRUD operations must use the last installed entity type and field storage definitions .

Proposed resolution

Mark those field storages with `custom_storage = TRUE`.

Remaining tasks

Review.

User interface changes

Nope.

API changes

Nope.

Data model changes

Nope.

Release notes snippet

Nope.