Fix module description highlight test

client/cypress/e2e/frontend/modules/highlights.cy.ts

@@ -13,6 +13,7 @@ const getModule = (contents) => {
   return {
     intro: '<p>Introducing this great paragraph!</p>',
     highlights: [],
+    slug: moduleSlug,
     chapters: [
       {
         title: 'A Chapter',

client/src/graphql/cache.ts

@@ -53,6 +53,11 @@ const typePolicies = {
           return previous !== undefined ? previous : false;
         },
       },
+      highlights: {
+        merge(_existing: HighlightNode[], incoming: HighlightNode[]) {
+          return incoming;
+        },
+      },
     },
     keyFields: ['slug'],
   },