improvement: flow coverage

src/templates/index-template.test.js

@@ -1,3 +1,4 @@
+// @flow
 import React from 'react';
 import renderer from 'react-test-renderer';
 import { StaticQuery, useStaticQuery } from 'gatsby';
@@ -5,6 +6,7 @@ import IndexTemplate from './index-template';
 import siteMetadata from '../../jest/__fixtures__/site-metadata';
 import allMarkdownRemark from '../../jest/__fixtures__/all-markdown-remark';
 import pageContext from '../../jest/__fixtures__/page-context';
+import type { RenderCallback } from '../types';
 
 describe('IndexTemplate', () => {
   const props = {
@@ -16,7 +18,7 @@ describe('IndexTemplate', () => {
 
   beforeEach(() => {
     StaticQuery.mockImplementationOnce(
-      ({ render }) => (
+      ({ render }: RenderCallback) => (
         render(siteMetadata)
       ),
       useStaticQuery.mockReturnValue(siteMetadata)