Issue #1839082 by Berdir: Implement the new entity field API for the field test field type.

core/modules/email/lib/Drupal/email/Tests/EmailItemTest.php

...es/field/lib/Drupal/field/Tests/FieldItemUnitTestBase.php

core/modules/field/lib/Drupal/field/Tests/ShapeItemTest.php

core/modules/field/lib/Drupal/field/Tests/TestItemTest.php

...ules/field/tests/modules/field_test/field_test.entity.inc

...dules/field/tests/modules/field_test/field_test.field.inc

...dules/field_test/lib/Drupal/field_test/Type/ShapeItem.php

...odules/field_test/lib/Drupal/field_test/Type/TestItem.php

...b/Drupal/taxonomy/Tests/TaxonomyTermReferenceItemTest.php