createContentPostUpgrade example

// Test the review form.     $this->assertReviewForm();

    $this->useTestMailCollector();
    $this->submitForm([], 'Perform upgrade');
    $this->assertUpgrade($this->getEntityCounts());

    \Drupal::service('module_installer')->install(['book']);

    // Test incremental migration.     $this->createContentPostUpgrade();

    $this->drupalGet('/upgrade');
    $session->pageTextContains("An upgrade has already been performed on this site. To perform a new migration, create a clean and empty new install of Drupal $this->destinationSiteVersion. Rollbacks are not yet supported through the user interface.");
    $this->submitForm([], 'Import new configuration and content from old site');
    $this->submitForm($this->edits, 'Review upgrade');
    $this->submitForm([], 'I acknowledge I may lose data. Continue anyway.');
    $session->statusCodeEquals(200);

    // Run the incremental migration and check the results.     $this->submitForm([], 'Perform upgrade');
    $this->assertUpgrade($this->getEntityCountsIncremental());
  }