jonathanfranks

    $fakeService = $this->createMock('Drupal\fake\Services\Fake');
    $fakeService->method('serviceMethod')
      ->withAnyParameters()
      ->willReturn(0);

    $container = new ContainerBuilder();
    $container->set('fake.service_name', $fakeService);
    \Drupal::setContainer($container);

jonathanfranks

  $migration_id = 'xxx';
  /** @var \Drupal\migrate\Plugin\MigrationInterface $migration */
  $migration = \Drupal::service('plugin.manager.migration')
    ->createInstance($migration_id);
  $migration->getIdMap()->prepareUpdate();
  $executable = new MigrateExecutable($migration, new MigrateMessage());
  $executable->import();