Method getMockForAbstractClass() is deprecated - replace in class EntityStorageBaseTest

Problem/Motivation

Method getMockForAbstractClass() of class PHPUnit\Framework\TestCase is deprecated in PHPUnit 10.

Some uses were removed as part of #3217904: [meta] Support PHPUnit 10 in Drupal 11, but there's a relevant number of places where this is still used.

Proposed resolution

replace in class EntityStorageBaseTest

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet