I have a method of a class that returns me the list of fields that have been created outside of core Drupal. The method is the following.
/**    * @param $contentType    * @return array $fields    */   public function getContentTypeFields($contentType) {     /** @var DrupalCoreEntityEntityFieldManagerInterface $entityManager */     $entityManager = Drupal::service('entity_field.manager');     $fields = array();      if (!empty($contentType)) {       $fields = array_filter(         $entityManager->getFieldDefinitions('node', $contentType),         function ($fieldDefinition) {           return $fieldDefinition instanceof FieldConfigInterface;         }       );     }      return $fields;   } 
It accepts a content type name and returns an array of fields that aren’t the default Drupal fields.
How can I write a unit Test for this method?