Make related items specs compatible w/ BootstrapVue 2

ee/spec/frontend/related_items_tree/components/related_items_tree_header_spec.js

@@ -30,6 +30,8 @@ const createComponent = ({ slots } = {}) => {
   });
 
   return shallowMount(RelatedItemsTreeHeader, {
+    attachToDocument: true,
+    sync: false,
     localVue,
     store,
     slots,

ee/spec/frontend/related_items_tree/components/tree_item_body_spec.js

@@ -46,6 +46,8 @@ const createComponent = (parentItem = mockParentItem, item = mockItem) => {
   });
 
   return shallowMount(TreeItemBody, {
+    attachToDocument: true,
+    sync: false,
     localVue,
     store,
     propsData: {