flow: Provide missing ReactTestRenderer type

Previously Flow did replace the undefined `ReactTestRenderer` type
with `any`. Not doing that anymore results in an error. So, just
import the correct type to fix the issue.

src/utils/tests.js

@@ -1,6 +1,7 @@
 /* @flow */
 import React from 'react';
 import { Provider } from 'react-redux';
+import type { ReactTestRenderer } from 'react-test-renderer';
 import renderer from 'react-test-renderer';
 
 import StylesProvider from '../boot/StylesProvider';