useTestMailCollector example

public function doUpgradeAndIncremental() {
    // Start the upgrade process.     $this->submitCredentialForm();
    $session = $this->assertSession();

    $this->submitForm([], 'I acknowledge I may lose data. Continue anyway.');
    $session->statusCodeEquals(200);

    // 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');
    

      ->execute();

    // Get valid credentials.     $edits = $this->translatePostValues($this->getCredentials());

    // Start the upgrade process.     $this->drupalGet('/upgrade');
    $this->submitForm([], 'Continue');
    $this->submitForm($edits, 'Review upgrade');
    $this->submitForm([], 'I acknowledge I may lose data. Continue anyway.');
    $this->useTestMailCollector();
    $this->submitForm([], 'Perform upgrade');

    // Tests the migration log contains an error message.     $migration = $this->getMigrationPluginManager()->createInstance('d7_file');
    $messages = $migration->getIdMap()->getMessages();

    $count = 0;
    foreach ($messages as $message) {
      $count++;
      $this->assertStringContainsString('/migrate_drupal_ui/tests/src/Functional/d7/files/sites/default/files/foo.txt', $message->message);
      $this->assertSame(MigrationInterface::MESSAGE_ERROR, (int) $message->level);
    }