Remove prefer-arrow-function error in test environment with mocha

[rewop]

Reading mocha documentation, they say to avoid the arrow function notation when defining tests because it breaks the context of mocha.

In the best scenario, we should be able to show an error when arrow function is used in such scenario.

[tkers]

If you look at the documentation of Mocha, you'll see that the lexical context is rarely needed to do something (I'm actually pretty sure that we have never used it before).

Whenever you try to use this in the a fat arrow function, ESLint will show an error about this anyway, so I don't really see the benefit of disallowing them altogether.