Added testArea for JS Unit tests

Added a testArea that is outside the viewport but still considered as
visible from the jQuery code.

The testArea is useful when:
- tested code tries to access DOM elements using global ids
- tested code is requiring some UI elements to be visible to activate
  themselves
Added unit tests for "add()" method for file upload

- Added OC.Upload.init() to make the code testable
- Added unit tests for the add() method of the uploader with some error
  cases
Fixed upload issue when free space is not known
Merge pull request #7592 from owncloud/fixstoragefreespace

Fixed upload issue when free space is not known (ex: FTP)