docs: use better mocha function types

[atjn]

If you use arrow functions, you cannot use mocha features like timeout and skip. The docs make it clear that they're not recommended: https://mochajs.org/#arrow-functions

Also the docs make it clear that you can use async functions in it to support async operations, but cannot do so in describe: https://mochajs.org/#using-async-await

A lot of people copy the example code and work from there. By using the correct function declarations and including async, you make it a lot easier to get started without encountering weird bugs or having to read a lot of mocha documentation.

Sister PR: #2924

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