EntityResourceTestBase::getNormalizedPostEntity() wrongly assumes that all entity types have auto-generated sequential IDs

Problem/Motivation

\Drupal\Tests\rest\Functional\EntityResource\EntityResourceTestBase::testPost() calls getNormalizedPostEntity() twice in order to get two valid representations of an entity for POST testing:

$parseable_valid_request_body   = $this->serializer->encode($this->getNormalizedPostEntity(), static::$format);
$parseable_valid_request_body_2 = $this->serializer->encode($this->getNormalizedPostEntity(), static::$format);

and there are two class variables $firstCreatedEntityId and $secondCreatedEntityId where we can specify the IDs for those two entities.

The problem is that calling getNormalizedPostEntity() twice without any possibility of returning different IDs per invocation assumes that the IDs are automatically generated by the DB layer. However, that's not the case for entity types with string IDs, where they are actually specified by the code which saves the entity (e.g. the entity form).

Proposed resolution

Add a EntityResourceTestBase::getSecondNormalizedPostEntity() method which allows test classes to specify a different, pre-determined ID for the second POST entity.

Remaining tasks

Review.

User interface changes

Nope.

API changes

Nope.

Data model changes

Nope.