Add Behat libraries to D8 core

The initiative to integration Behat into Drupal 8 core (for automated front-end testing) requires the addition of a new libraries to the core composer.json file.

Specifically:

must be added. This will in turn bring in the required Mink and Behat dependencies. This is a pre-requisite for the core Behat module, which is being worked on in #2232271: [Meta] Use Behat for validation testing.