Make issuables list specs compatible w/ BootstrapVue 2

spec/frontend/issuables_list/components/issuable_spec.js

@@ -45,6 +45,7 @@ describe('Issuable component', () => {
         ...props,
       },
       sync: false,
+      attachToDocument: true,
     });
   };
 

spec/frontend/issuables_list/components/issuables_list_app_spec.js

@@ -49,6 +49,7 @@ describe('Issuables list component', () => {
       },
       localVue,
       sync: false,
+      attachToDocument: true,
     });
   };