Fixing up scenarios for managing page permissions Behat tests.

Relies on pull requests from silverstripe-behat-extension, so any Travis tests will fail without them.

silverstripe-labs/silverstripe-behat-extension#15
silverstripe-labs/silverstripe-behat-extension#14
silverstripe-labs/silverstripe-behat-extension#13
silverstripe-labs/silverstripe-behat-extension#12