Unit test the Drupal\serialization\Normalizer\EntityNormalizer class

1. +++ b/core/modules/serialization/tests/Drupal/serialization/Tests/Normalizer/EntityNormalizerTest.php
   @@ -0,0 +1,187 @@
   +/**
   + * Tests the EntityNormalizer class.
   + */
   +class EntityNormalizerTest extends UnitTestCase {
   ...
   +  /**
   +   * Tests the normalize() method.
   +   */
   +  public function testNormalize() {
   ...
   +  /**
   +   * Tests the denormalize method with a bundle property.
   +   */
   +  public function testDenormalizeWithBundle() {
   ...
   +  /**
   +   * Tests the denormalize method with no bundle defined.
   +   */
   +  public function testDenormalizeWithNoBundle() {
   

   It would be great to use @coversDefaultClass and @covers

2. +++ b/core/modules/serialization/tests/Drupal/serialization/Tests/Normalizer/EntityNormalizerTest.php
   @@ -0,0 +1,187 @@
   +
   +    $content_entity = $this->getMockBuilder('Drupal\Core\Entity\ContentEntityBase')
   +      ->disableOriginalConstructor()
   +      ->setMethods(array('getProperties'))
   +      ->getMockForAbstractClass();
   

   I try to figure out why we cannot simply mock the ContentEntityInterface here? Oh right this bullshit transverable thing.

Thank you

Committed 8f546e5 and pushed to 8.x. Thanks!