ftest fix incorrect nuxeo-dropzone input selector

The setter for the nuxeo-dropzone widget doesn't select the input inside the nuxeo-dropzone element passed as a parameter. Instead it uses browser.waitForExist("nuxeo-dropzone input[id='input']") which may return the input of another nuxeo-dropzone should there be several in the DOM.

See https://github.com/nuxeo/nuxeo-web-ui/blob/fd3b2d184a0653e967198281e5dcfea29eeab0c4/packages/nuxeo-web-ui-ftest/features/step_definitions/support/fixtures/layouts.js#L203

Same issue for the getter