New rule: No arrow function expression as test callback

[mitchlloyd]

As arrow functions become more commonly used, developers may use them to create QUnit test callbacks. This changes the expected test context to something else (probably to window). Some testing setups have specialized modules that use this inside beforeEach to manipulate the context, so changing it can be problematic.

Example that should warn:

test('description', (assert) => {
});

Example that is ok:

test('description', function(assert) {
});

I haven't thought too much about implementation yet, but it seems fairly simple to look for the test callback and then ensure that an arrowFunctionExpression is not being used.

It might also make sense to apply this rule to the beforeEach and afterEach callbacks in module setup.

[platinumazure]

Excellent call, agree that it should be on test and module callbacks, consider this accepted (until I can get to a desktop and apply the label).

Thoughts on rule name? I'm thinking no-arrow-tests, but maybe you can do better, I'm feeling meh. Feel free to submit a PR

[platinumazure]

I'll start work on this.

@mitchlloyd Do you think there should be an option to tolerate arrow functions if they don't refer to this? In theory, that shouldn't cause problems. (I would still want the default option to be outright forbidding arrow functions as test callbacks, though.)

[mitchlloyd]

I don't have any interest in allowing people to use both => and function for defining test callbacks. I just don't see any advantage to allowing the => style.

[platinumazure]

@mitchlloyd I'm with you on that-- I'm just trying to figure out if there might be others who would want the linter to only flag it when someone is actually relying on this in an arrow function.

It's easy to add this later if someone wants it, so I guess for now I'll go with no arrow functions period. In that case, I'm pretty close to having a branch ready.

[platinumazure]

I've put up a new branch. Could you take a look at the documentation in that branch to make sure I haven't misrepresented anything (especially about arrow functions, which I don't regularly use)? Thanks!