Issue #2073871 by nick_schuch: TourTestBasic should setup Seven has the default admin theme.

core/modules/tour/lib/Drupal/tour/Tests/TourTestBasic.php

...dules/tour/tests/tour_test/config/tour.tour.tour-test.yml

.../modules/tour/tests/tour_test/tour_test.local_actions.yml

core/modules/tour/tests/tour_test/tour_test.module

core/modules/tour/tests/tour_test/tour_test.routing.yml