Use installConfig() in our Kernel tests

We have this code in (almost) all of our Kernel tests:

    // Manually set the tracking page size since the module's default
    // configuration is not installed automatically in kernel tests.
    \Drupal::configFactory()
      ->getEditable('search_api.settings')
      ->set('tracking_page_size', 100)
      ->save();

However, we can instead just install our default configuration with $this->installConfig('search_api');, which would be self-explanatory, a lot shorter and also more future-proof (if we ever need another default config value).