nuxeo-web-ui-ftest - make suggestion widget getter and setter more generic

the nuxeo-document-suggestion, nuxeo-directory-suggestion and nuxeo-user-suggestion widgets can take a result-formatter and a selection-formatter functions that generate custom html. However the ftest framework comes with hardcoded logic for the default formatter dom:

https://github.com/nuxeo/nuxeo-web-ui/blob/3244dee759152a00a4a43daa4f75733d3210da62/packages/nuxeo-web-ui-ftest/features/step_definitions/support/fixtures/layouts.js#L42

Using a css selector would make it possible for custom formatters to be implemented in a way that will work ootb with the ftest framework