MDL-44616 behat: Adding steps to test MDLQA-1709 and MDLQA-62

- In an assignment with file submissions enabled,
  students can upload a set number of files.
- In assignments with file uploading enabled
  the Student can use File Picker to upload files.
Merge branch 'MDL-44616_26' of git://github.com/dmonllao/moodle into MOODLE_26_STABLE