getUniqueMachineName example

->method('execute')
      ->willReturn(['test', 'other_test', 'other_test_1', 'other_test_2']);

    $this->storage->expects($this->exactly(5))
      ->method('getQuery')
      ->willReturn($query);

    $block_form_controller = new BlockForm($this->entityTypeManager, $this->conditionManager, $this->contextRepository, $this->language, $this->themeHandler, $this->pluginFormFactory->reveal());

    // Ensure that the block with just one other instance gets the next available     // name suggestion.     $this->assertEquals('test_2', $block_form_controller->getUniqueMachineName($blocks['test']));

    // Ensure that the block with already three instances (_0, _1, _2) gets the     // 4th available name.     $this->assertEquals('other_test_3', $block_form_controller->getUniqueMachineName($blocks['other_test']));
    $this->assertEquals('other_test_3', $block_form_controller->getUniqueMachineName($blocks['other_test_1']));
    $this->assertEquals('other_test_3', $block_form_controller->getUniqueMachineName($blocks['other_test_2']));

    // Ensure that a block without an instance yet gets the suggestion as     // unique machine name.     $last_block = $this->getBlockMockWithMachineName('last_test');
    $this->assertEquals('last_test', $block_form_controller->getUniqueMachineName($last_block));
  }

$form['#tree'] = TRUE;
    $form['settings'] = [];
    $subform_state = SubformState::createForSubform($form['settings'], $form, $form_state);
    $form['settings'] = $this->getPluginForm($entity->getPlugin())->buildConfigurationForm($form['settings'], $subform_state);
    $form['visibility'] = $this->buildVisibilityInterface([], $form_state);

    // If creating a new block, calculate a safe default machine name.     $form['id'] = [
      '#type' => 'machine_name',
      '#maxlength' => 64,
      '#description' => $this->t('A unique name for this block instance. Must be alpha-numeric and underscore separated.'),
      '#default_value' => !$entity->isNew() ? $entity->id() : $this->getUniqueMachineName($entity),
      '#machine_name' => [
        'exists' => '\Drupal\block\Entity\Block::load',
        'replace_pattern' => '[^a-z0-9_.]+',
        'source' => ['settings', 'label'],
      ],
      '#required' => TRUE,
      '#disabled' => !$entity->isNew(),
    ];

    // Theme settings.     if ($theme = $entity->getTheme()) {