Add test coverage for file entity reference selection plugin

Thinking I got tickets crossed in #28?

But ran #27 for D10 tests and had CI failures.

- Updated a new kernel test to validate the entity query results.
- Using other general practices for creating fields etc.

Code is already fixed so no changes are required in EntityReference handler plugin.
Hiding existing patches in favour of MR.

Tests are passing on local.

Thanks for working on this!

Test failures seem related to this change.