TestSelection example

'info' => 'Non-reusable Block',
      'type' => 'spiffy',
      'reusable' => FALSE,
    ]);
    $this->blockNonReusable->save();

    $configuration = [
      'target_type' => 'block_content',
      'target_bundles' => ['spiffy' => 'spiffy'],
      'sort' => ['field' => '_none'],
    ];
    $this->selectionHandler = new TestSelection($configuration, '', '', $this->container->get('entity_type.manager'), $this->container->get('module_handler'), \Drupal::currentUser(), \Drupal::service('entity_field.manager'), \Drupal::service('entity_type.bundle.info'), \Drupal::service('entity.repository'));

    // Setup the 3 expectation cases.     $this->expectations = [
      'both_blocks' => [
        'spiffy' => [
          $this->blockReusable->id() => $this->blockReusable->label(),
          $this->blockNonReusable->id() => $this->blockNonReusable->label(),
        ],
      ],
      'block_reusable' => ['spiffy' => [$this->blockReusable->id() => $this->blockReusable->label()]],
      'block_non_reusable' => ['spiffy' => [$this->blockNonReusable->id() => $this->blockNonReusable->label()]],
    ];