Remove entity_load* usage for entity_test, entity_test* entity types

Parent Issue

#2359437: [meta] Remove entity_load* family of functions usage from the code base

Problem/Motivation

There are case where entities of the entity_test family of entity types are retrieved using the functional loader, entity_load*()

The entity types to be changed are:

• entity_test
• entity_test_rev
• entity_test_ml
• entity_test_mulrev

These entity types are handled in one sub-issue, because they are tightly related, and are exposed by the same module

Proposed resolution

Replace calls to entity_load*('entity_test*', ...) with EntityTest*::load*() or by using \Drupal::entityTypeManager()->getStorage('entity_test*')->load*(...)