BUG Fix deleted pages redirecting the CMS

Update behat tests for Mink 1.6 compatibility

Fixes issue with viewing of archived (deleted) pages in the cms preview pane which was breaking behat tests

Relies on https://github.com/silverstripe-labs/silverstripe-behat-extension/pull/47 to be merged first
@tractorcow Why did this change from "viewer" to "editor"?