Fix shared items object in nuxeo-page-provider-display-behavior

The items object on nuxeo-page-provider-display-behavior is shared between all instances until it is set again. This causes some problems when testing. It can be fixed with:

      items: {
        type: Array,
        value() {
          return [];
        },
        notify: true,
      },

But it might have side effects and must be tested first.