Implement Jest's test framework setup configuration file

The setup documentation (1) provides instruction for best practices on
how to configure the library for a optimal and easy developer workflow.
This includes a global configuration that has been placed within the
`test` base directory named `setup.js` and will be loaded via Jest's
`setupTestFrameworkScriptFile` option .
Initially it imports the `react-testing-library/cleanup-after-each`
script which will automatically execute `afterEach(cleanup)` for each
test which prevents unnecessary boilerplate per test file and possible
problems when the function is not called accidentally. It also imports
`jest-dom/extend-expect` to automatically extend the `expect` global
with jest-dom's custom matchers (2).

References:
  (1) https://github.com/kentcdodds/react-testing-library#setup
  (2) https://github.com/gnapse/jest-dom#custom-matchers

GH-39

test/setup.js

@@ -0,0 +1,29 @@
+/*
+ * Copyright (C) 2018-present Arctic Ice Studio <[email protected]>
+ * Copyright (C) 2018-present Sven Greb <[email protected]>
+ *
+ * Project:    Nord Docs
+ * Repository: https://github.com/arcticicestudio/nord-docs
+ * License:    MIT
+ */
+
+/**
+ * @file The setup to configure Jest's test framework before each test.
+ * @author Arctic Ice Studio <[email protected]>
+ * @author Sven Greb <[email protected]>
+ * @see https://jestjs.io/docs/en/configuration
+ * @see https://www.gatsbyjs.org/docs/testing
+ * @since 0.1.0
+ */
+
+/*
+ * Extend the `expect` global with jest-dom's custom matchers.
+ *
+ * @see https://github.com/gnapse/jest-dom#custom-matchers
+ */
+import "jest-dom/extend-expect";
+
+/*
+ * Automatically execute `afterEach(cleanup)` for each test.
+ */
+import "react-testing-library/cleanup-after-each";