Add enzyme for testing

Change-Id: If64eba12b5c255e4d0143eab009499793674778a

src/App.test.js

@@ -0,0 +1,14 @@
+import React from "react";
+import { TestContext } from "react-admin";
+import { shallow } from "enzyme";
+import App from "./App";
+
+describe("App", () => {
+  it("renders", () => {
+    shallow(
+      <TestContext>
+        <App />
+      </TestContext>
+    );
+  });
+});