Adding Jest for testing

src/__mocks__/gatsby.js

@@ -0,0 +1,14 @@
+const React = require('react');
+const gatsby = jest.requireActual('gatsby');
+
+module.exports = {
+  ...gatsby,
+  graphql: jest.fn().mockImplementation(([query]) => query),
+  Link: jest.fn().mockImplementation(({ to, ...rest }) =>
+    React.createElement('a', {
+      ...rest,
+      href: to
+    })
+  ),
+  StaticQuery: jest.fn()
+};