Include "jest-styled-components" in test framework script file

This commit integrates jest-styled-components (1), a set of utilities
for testing "Styled Components" (2) with Jest () that improves the
napshot testing experience and provides a brand new matcher to make
expectations on the style rules.
This is the officially recommended library styled-components for Jest
integration (3).

Instead of storing the generated class names in snapshots it allows to
track the actual CSS rules and attributes for better comparison what
changed and if the change is even really related to the affected
component.

> Configuration

To enable this features the package is simply imported in the test
framework setup file defined in Jest's `setupTestFrameworkScriptFile`
field (GH-39).

> Custom Style Matcher

The custom `toHaveStyleRule` matcher (4) is useful to test if a given
rule is applied to a component. It will be added to the extended global
`except` object when the main package file is being imported.

References:
  (1) https://github.com/styled-components/jest-styled-components
  (2) https://www.styled-components.com
  (3) https://jestjs.io
  (4) https://github.com/styled-components/jest-styled-components#tohavestylerule

Associated epics: GH-38
Associated issues: GH-39
GH-76

test/setup.js

@@ -13,6 +13,9 @@
  * @author Sven Greb <[email protected]>
  * @see https://jestjs.io/docs/en/configuration
  * @see https://www.gatsbyjs.org/docs/testing
+ * @see https://github.com/gnapse/jest-dom#custom-matchers
+ * @see https://github.com/styled-components/jest-styled-components#tohavestylerule
+ * @see https://www.styled-components.com/docs/tooling#jest-integration
  * @since 0.1.0
  */
 
@@ -27,3 +30,12 @@ import "jest-dom/extend-expect";
  * Automatically execute `afterEach(cleanup)` for each test.
  */
 import "react-testing-library/cleanup-after-each";
+
+/*
+ * Add support and utilities for testing styled-somponents with Jest.
+ *
+ * @see https://github.com/styled-components/jest-styled-components#tohavestylerule
+ * @see https://www.styled-components.com/docs/tooling#jest-integration
+ * @since 0.3.0
+ */
+import "jest-styled-components";