Move helpers in entity_test.module and delete it

Problem/Motivation

.module files are only necessary when there are preprocess functions now that almost all hooks can be OOP

Steps to reproduce

Open file

Proposed resolution

• entity_test_form_entity_test_form_validate() to Callbacks
• entity_test_form_entity_test_form_validate_check() to Callbacks
• entity_test_field_default_value() to Callbacks
• _entity_test_record_hooks() to EntityTestHooks
• Add return type to moved functions.

Will be done in separate issue:

• move entity_test_create_bundle(), entity_test_delete_bundle() to EntityTestHelper #3495966: Deprecate and replace entity_test_create_bundle(), entity_test_delete_bundle()
• move entity_test_entity_types() to EntityTestHelper, convert const to enum #3495968: Move entity_test_entity_types from entity_test.module

Remaining tasks

None

User interface changes

N/A

Introduced terminology

N/A

API changes

N/A

Data model changes

N/A

Release notes snippet

N/A