Set up Unit Testing environment with Gatsby, Jest and @testing-library/react; added some basic unit tests for Castform template

__mocks__/gatsby.js

@@ -0,0 +1,15 @@
+const React = require(`react`);
+const gatsby = jest.requireActual(`gatsby`);
+
+module.exports = {
+  ...gatsby,
+  graphql: jest.fn(),
+  Link: jest.fn().mockImplementation(({ to, ...rest }) =>
+    React.createElement(`a`, {
+      ...rest,
+      href: to,
+    }),
+  ),
+  StaticQuery: jest.fn(),
+  useStaticQuery: jest.fn(),
+};