PHPUnit testing within multisite setups

As a workaround, for Kernel tests, you can do this:

  /**
   * {@inheritdoc}
   */
  protected function setUpFilesystem(): void {
    parent::setUpFilesystem();
    $this->setSetting('test_parent_site', 'sites/SITENAME');
  }

for Functional tests:

  /**
   * {@inheritdoc}
   */
  protected function prepareSettings() {
    parent::prepareSettings();
    $directory = DRUPAL_ROOT . '/' . $this->siteDirectory;
    $append = '$settings["test_parent_site"] = "sites/SITENAME";';
    file_put_contents($directory . '/settings.php', $append, FILE_APPEND | LOCK_EX);
  }

this worked for me to get my tests moving.