Addresses issue #11 items unit testing, snapshot testing. Follows instructions from https://www.gatsbyjs.com/docs/how-to/testing/unit-testing/ ; Adding typescript config, mocks, snapshot capabilities; initial test for j40footer and snapshot, readme update, test helper for intl plugin (#79)

client/src/components/J40Footer.spec.tsx

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+import React from 'react';
+import renderer from 'react-test-renderer';
+import J40Footer from './J40Footer';
+import {LocalizedComponent} from '../test/testHelpers';
+
+describe('J40Footer', () => {
+  it('renders correctly', () => {
+    const tree = renderer
+        .create(<LocalizedComponent><J40Footer /></LocalizedComponent>)
+        .toJSON();
+    expect(tree).toMatchSnapshot();
+  });
+});

client/src/components/__snapshots__/J40Footer.spec.tsx.snap

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+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`J40Footer renders correctly 1`] = `
+Array [
+  <footer
+    className="usa-footer"
+  >
+    <div
+      className="usa-footer__primary-section"
+    />
+    <div
+      className="usa-footer__secondary-section"
+    >
+      <div
+        className="grid-container"
+      >
+        <a
+          href="/en/"
+          onClick={[Function]}
+        >
+          Home
+        </a>
+      </div>
+    </div>
+  </footer>,
+  ",",
+]
+`;

client/src/test/testHelpers.tsx

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+import React, {ReactNode} from 'react';
+import {IntlContextProvider} from 'gatsby-plugin-intl';
+
+interface ILocalizedComponentProps {
+  children: ReactNode
+}
+
+export const LocalizedComponent = ({children}: ILocalizedComponentProps) => {
+  return (
+    <>
+      <IntlContextProvider value={{language: 'en', routed: true}}>
+        {children}
+      </IntlContextProvider>,
+    </>
+  );
+};