UpdatePathTestBase::prepareSettings writes entity_update_batch_size incorrectly

Problem/Motivation

Discovered here https://git.drupalcode.org/project/drupal/-/merge_requests/5500#note_235984

In UpdatePathTestBase::prepareSettings it tries to override entity_update_batch_size to 1 so that all update batches that use the setting process 1 thing at a time. However, debugging through ::writeSettings I saw that $buffer simply wrote $entity_update_batch_size = 1; to the settings file. This is because we are missing a settings array key.

Steps to reproduce

Run an update test that hits an update hook that uses Settings::get('entity_update_batch_size') and notice it's not 1.

Proposed resolution

Fix array key.

Remaining tasks

Commit