Investigate why tests now take twice as long to run

After #2154225: Switch Behat widget tests to use "Landing page", our tests have started taking about twice as long on Travis-CI. We even hit the 50 minute limit on a couple jobs!

I suspect it's because of the cache clear in FeatureContext::bootstrapDrupal():

  /**
   * @BeforeScenario @api
   *
   * Bootstrap Drupal so that all Drupal API functions work.
   */
  public function bootstrapDrupal($event) {
    if (!$this->bootstrapped) {
      $drupal = $this->getDriver('drupal');

      // We are experiencing a weird issue where the CTools plugin cache gets
      // corrupted and none of our Panels pages work because their layout
      // plugins can't be found. It appears that clearing the cache, and then
      // bootstrapping Drupal again will fix it! I suspect the CTools plugin
      // cache is being built too early, ie. before we change to the Drupal
      // directory - but I haven't been able to confirm that.
      $drupal->clearCache();
      $drupal->bootstrap();

      // We occasionally get errors about not finding the ctools_get_export_ui()
      // function, so we force it to be loaded. No idea what's causing this.
      ctools_include('export-ui');

      $this->bootstrapped = TRUE;
    }
  }

However, the tests won't work at all with out that. :-/

Once we confirm this is the problem, there are two options I can think of right now:

1. Come up with a way to only clear the CTools plugin cache, rather than clearing all the caches, or
2. Running the tests from the Drupal directory, so that it never has trouble finding the CTools plugins