Create functional tests for EB integration with IEF

This is a child issue of #2809159: [META] Improve test coverage in entity browser, specifically for the part:

Integration with Inline Entity Form

• IEF use existing (partly tested in InlineEntityIntegrationTest.php, test full flow: entities are selected, IEF table is automatically reloaded, correct entities appear in it)
• High prio: Entity browser on a field inside IEF
• High prio: Entity browser on a field inside nested IEF (2+ levels up)
• High prio: Entity browser on a field inside IEF, more than one IEF field on the entity (open EB in one, update, open in the other, update, have both open at the same time, update, make sure referenced entities are correct - they are not leaking between fields)