Fix System Tests in CI environment

Resolve breaking changes introduced in `@hotwired/turbo` that were later
fixed in [@hotwired/turbo#4f334da][] (and further resolved in
[@hotwired/turbo#7a7c6e2][]).

CI is failing because those changes are not available as part of a beta
release. Despite the fact that these changes wouldn't be necessary if
that release were available, this resolves the broken build pipeline
more immediately.

[@hotwired/turbo#4f334da]: hotwired/turbo@4f334da
[@hotwired/turbo#7a7c6e2]: hotwired/turbo@7a7c6e2

test/dummy/app/javascript/application.js

@@ -1,2 +1,4 @@
 // Configure your import map in config/importmap.rb. Read more: https://github.com/rails/importmap-rails
-import "@hotwired/turbo-rails"
+import { Turbo } from "@hotwired/turbo-rails"
+
+window.Turbo = Turbo