Add with-mocha example

[msutkowski]

Changes

• Adds a minified example from the Mocha issue thread

Example checklist

• I've followed the Contribution guidelines.
• I've verified that all added tests pass by running npm test.
• I've included only the necessary minimum setup to showcase the example.
• I've added a README.md file to the root directory of my example.

[msutkowski]

I left this pretty close to the example issue and solution. I think this is general enough to show as an example for mocha users and explains the caveats (no parallel mode support as of now)

[kettanaito]

Do we need the localstorage-polyfill if we use whatwg-fetch instead?

I find it the less things our examples have, the more straightforward they are to follow.

[msutkowski]

@kettanaito localstorage would still need to be mocked or polyfilled here using whatwg-fetch. I can just include a mock if you think it's more transparent than including this polyfill or a mock lib, just let me know :)

[kettanaito]

Doesn't RTL call cleanup automatically? Does that work only in Jest?

[msutkowski]

It should, but unfortunately, that's not the case here. If you add console.log(screen.debug()) to the last test in Child, you'll see that it stacks components and the tests will fail.

[kettanaito]

Thanks for preparing this example, @msutkowski! I've left a few comments.

[msutkowski]

@kettanaito Updated, but left the localstorage polyfill in place per the comment. If you want me to include a manual mock for this, just let me know :)

[kettanaito]

Thanks for all the work on this, @msutkowski! As I'm updating the examples, I think I will drop this one. The usage with mocha is rather straightforward, and mocha+react is a bit too complex to feature as a standalone example.