Disable secret key validation on admin noroute to fix redirect loop

[convenient]

Similar to 2f422ad but using the framework.

Fixes issue #10611 for the 2.2-develop branch.

Doesn't address the route cause, but only fixes the symptom as per #10921 (comment)

If preferred I can rebase this branch to do a cherry-pick of 2f422ad, but I figured it better to use the framework to achieve this feature.

Steps to reproduce

1. Create a user role with no resources assigned to it.
2. Create an admin user with that role.
3. Log in with that user.
4. Should see an "Access denied" screen rather than a redirect loop.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[convenient]

Runtime:       PHP 7.1.9
Configuration: /home/travis/build/magento/magento2/dev/tests/functional/phpunit.xml
...............
The job exceeded the maximum time limit for jobs, and has been terminated.

@okorshenko Any chance we can re-trigger the travis build to see if this is some intermittent issue?

[okorshenko]

Done