Using styled-jsx for with-jest example. (#1052)

examples/with-jest/__tests__/__snapshots__/index.test.js.snap

 exports[`With Snapshot Testing App shows "Hello world!" 1`] = `
-<div>
-  <p>
+<div
+  data-jsx={2648947580}>
+  <p
+    data-jsx={2648947580}>
     Hello World!
   </p>
 </div>

examples/with-jest/pages/index.js

 export default () => (
   <div>
+    <style jsx>{`
+      p {
+        color: red;
+      }
+    `}</style>
     <p>Hello World!</p>
   </div>
 )