In Drupal 7, when developing a test that extended DrupalWebTestCase, each test function would automatically install a new version of the site using the standard profile.
In Drupal 8, this was changed to the 'testing' profile instead; a low-weight profile that doesn't install any non-required modules/themes/etc.
If your tests have assumptions in them about things such as Block module being enabled ("undefined permission 'administer blocks'") or Bartik theme being used instead of Stark, add the following to the top of your test class definition:
class TextSummaryTest extends WebTestBase {
protected $profile = 'standard'; // This line here.
Try to aim to reduce these assumptions, however, because Testing profile is much faster. Instead, add the required dependencies to to your test directly.