Make Tests 'multi[form][entity_id]' agnostic

As found out in Unable to keep nested IEF data separate with multivalue fields. there's another HTML ID that we need to make more agnostic of its exact location:

$assert_session->waitForField('multi[form][entity_id]')

That is killing the testing of the mentioned issue.

Let's see if we can get rid of it and replace it with something more along the lines of Make tests (more) HTML ID agnostic