Add Jest testing skeleton and one snapshot test (#613)

v2/__mocks__/file-mock.js

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+module.exports = "test-file-stub"

v2/__mocks__/gatsby.js

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+const React = require("react")
+const gatsby = jest.requireActual("gatsby")
+
+module.exports = {
+  ...gatsby,
+  graphql: jest.fn(),
+  Link: jest.fn().mockImplementation(
+    ({
+      activeClassName,
+      activeStyle,
+      getProps,
+      innerRef,
+      partiallyActive,
+      ref,
+      replace,
+      to,
+      ...rest
+    }) =>
+      React.createElement("a", {
+        ...rest,
+        href: to,
+      })
+  ),
+  StaticQuery: jest.fn(),
+  useStaticQuery: jest.fn(),
+}

v2/__mocks__/i18n/provider.js

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+module.exports = {
+  useLocaleContext: jest.fn(() => ({
+    slug: "",
+  })),
+}

v2/jest.config.js

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+module.exports = {
+  moduleNameMapper: {
+    ".+\\.(css|styl|less|sass|scss)$": `identity-obj-proxy`,
+    ".+\\.(jpg|jpeg|png|gif|eot|otf|webp|svg|ttf|woff|woff2|mp4|webm|wav|mp3|m4a|aac|oga)$": `<rootDir>/__mocks__/file-mock.js`,
+  },
+  testPathIgnorePatterns: [`node_modules`, `\\.cache`, `<rootDir>.*/public`],
+  transformIgnorePatterns: [`node_modules/(?!(gatsby)/)`],
+  globals: {
+    __PATH_PREFIX__: ``,
+  },
+  testURL: `http://localhost`,
+  setupFiles: [`<rootDir>/jest/loadershim.js`],
+}

v2/jest/loadershim.js

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+global.___loader = {
+  enqueue: jest.fn(),
+}