Make MR widget approvals summary spec compatible w/ BootstrapVue 2

ee/spec/frontend/vue_mr_widget/components/approvals/approvals_summary_optional_spec.js

@@ -18,6 +18,7 @@ describe('EE MRWidget approvals summary optional', () => {
       propsData: props,
       sync: false,
       localVue,
+      attachToDocument: true,
     });
   };