MDL-38184 behat: Fields management externalized

Not only behat_forms needs to deal
with form fields
MDL-38184 behat: File-related actions
MDL-38184 behat: Solving problem with the tests loader
MDL-38184 behat: Filepicker management steps

Includes create/open/delete/zip/unzip
of files and folders
MDL-38184 behat: Tests for filepicker steps
MDL-38184 repository_upload: New step definition
MDL-38184 repository_upload: Adding tests