ci: minimum working example for jest-axe accessibility CI

[mattxwang]

This PR implements the minimum overhead required to run jest-axe via GitHub Actions on a set of predefined components. In order, we:

1. install jest, jest-axe, @testing-library/react, @testing-library/jest-dom, the minimum set up for a viable test infrastructure
   1. to install jest, we add a general jest.config.js as a jest configuration file, and jest.setup.js as a pre-test setup file (automatically importing our dependencies)
   2. creates a set of mock module mappers for "non-code" webpack elements; a stub file mock (__mocks__/fileMock.js) for most files, and the identity-obj-proxy to proxy-back CSS/SASS/SCSS class names
   3. adds the next preset to babel
   4. adjusts the ESLint config to use the jest environment
2. implements a rudimentary accessibility test in NewsArticle.test.js, which is a simple subcomponent with no errors. You can validate this working by adding an arbitrary <img /> to NewsArticle.js; npm test will throw an error.
3. implements a rudimentary failing accessibility test in Footer.test.js, where an <ul> has children that are not <li> - showing that negatives work. I then resolve this issue.
4. updates the "Node.JS CI" action to be "Build and Test", and runs npm test in it

Ideally, you should be able to add more test files following the configuration in NewsArticle.test.js; note the strange let pattern with the async version of act.

I didn't use the native next testing plugin, since that seems blocked by the upgrade to Next 12 (#409).

Part of #218, supersedes #301.

Relevant reading:

• Configuring Jest
• act() in React
• Blog Post: Next.js 11: Setup & Config for Testing, Linting, and Absolute Imports

[mattxwang]

Note that I've introduced a failing test for footer in 03bebdc, and then resolved it in 9a90696 - indicating that the test pattern works.

[advaithg]

Looks good!