wait for mask to disapear on big pages

In order to avoid this kind of error :
An error occurred in an after hook
  Capybara::Poltergeist::MouseEventFailed: Firing a click at co-ordinates [1037, 20] failed. Poltergeist detected another element with CSS selector 'html body form div.controls div.chooser-container div.chooser-mask' at this position. It may be overlapping the element you are trying to interact with. If you don't care about overlapping elements, try using node.trigger('click').