Run Publishing E2E Tests as part of a build

This change will cause [Publishing End-to-end Tests][e2e-tests] to run
once the initial unit tests for this build has passed and will be
triggered as a dependent build.

These are used to test that none of the changes made in a commit to this
repo don't break the tech stack or break the end-to-end tests
environment.

The End-to-end tests take approximately 4/5 minutes to run and will make
it slower to get a passing build.

[e2e-tests]: https://github.com/alphagov/publishing-e2e-tests

Jenkinsfile

@@ -19,5 +19,6 @@ node {
         govuk.runTests("spec:javascript")
       }
     },
+    publishingE2ETests: true,
   )
 }