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.

MR is green now.
Moving back to NR

Thanks for fixing that, seems like good test coverage. LGTM.

The Needs Review Queue Bot → tested this issue. It no longer applies to Drupal core. Therefore, this issue status is now "Needs work".

This does not mean that the patch necessarily needs to be re-rolled or the MR rebased. Read the Issue Summary, the issue tags and the latest discussion here to determine what needs to be done.

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Moving back to RTBC since the MR is already green.