Update WritingTests to not use arrow functions

[mlippens]

Mocha advises against using arrow functions, so it seems wrong to me to use them in WritingTests.md as an example.

[SimenB]

Why? Because of this?

[mlippens]

That's what they say in the documentation of mocha, though myself I have not run into any issues with it so far.

[rosskevin]

Mocha doesn't want you to use arrow functions only so that you can access the mocha instance to set expirations etc, otherwise, arrow functions work just fine. I expect Mocha will need to adapt that approach soon as since the arrow fn syntax is being adopted quickly and since node 6 is out.

[SimenB]

Node 4 supports arrow functions as well

[GGAlanSmithee]

If you need the Mocha context, only the outer most function must not be an arrow function.