Kernel

Assert tags

 /**
   * Tests the cache tags from image formatters.
   */
  public function testImageFormatterCacheTags() {

    // Create a test entity with the image field set.
    $entity = EntityTest::create([
      'name' => $this->randomMachineName(),
    ]);
    $entity->{$this->fieldName}->generateSampleItems(2);
    $entity->save();

    // Generate the render array to verify if the cache tags are as expected.
    $build = $this->display->build($entity);
    $this->assertEquals($entity->{$this->fieldName}[0]->entity->getCacheTags(), $build[$this->fieldName][0]['#cache']['tags'], 'First image cache tags is as expected');
    $this->assertEquals($entity->{$this->fieldName}[1]->entity->getCacheTags(), $build[$this->fieldName][1]['#cache']['tags'], 'Second image cache tags is as expected');
  }

Note: See also assertcachetags method.

Reference

1. ImageFormatterTest