Regression: lost test coverage for handling default images in the Image field

Problem/Motivation

While writing a test for #2322421: Recoverable fatal error: Object of class Drupal\Core\Field\FieldItemList I noticed a very peculiar line in ImageFieldDefaultImagesTest:

foreach (['field', 'field', 'field2', 'field_new', 'field_new'] as $image_target) {

Surely someone who can write a test knows that duplicating array keys doesn't lead to anything useful, so I dug a bit in the history of that file and it looks like #2312093: Rename FieldInstanceConfig to FieldConfig is the culprit. The original line looked like this in D7:

foreach (array('field', 'instance', 'instance2', 'field_new', 'instance_new') as $image_target) {

which, in D8 terms, means: ['field_storage', 'field', 'field2', 'field_storage_new', 'field_new'].

The consequence is that we've lost half of the test coverage from that test class, the half that deals with checking different default images being set on the field storage config vs. field confg and the fallback to the field storage setting if the field doesn't specify a default image.

Proposed resolution

Let's fix it :)

Remaining tasks

Review.

User interface changes

Nope.

API changes

Nope.

Data model changes

Nope.