Minor issue in EntityReferenceRelationshipTest

Follow-up to #1906806: Provide a relationship for each entity reference field.

Problem/Motivation

While working on #2321721: Provide a views relationship for each dynamic entity reference field @larowlan pointed this issue to me.
Drupal\entity_reference\Tests\Views\EntityReferenceRelationshipTest

    $entity = $entity_storage->create(array('field_test' => $entity->id()));
    $entity->field_test->target_id = $referenced_entity->id();
    $entity->save();

So field_test value is overwritten by 2nd line.

Proposed resolution

See the attached patch

Remaining tasks

Review

User interface changes

None

API changes

None