Detect missing assertions in an arrow body

[jonathanperret]

I stumbled upon a project that was missing expect assertions and tried to use eslint-plugin-chai-expect to find all occurrences but due to the way they wrote some of their tests (https://github.com/codeceptjs/CodeceptJS/blob/04739e52ddd725fc516a691b77ec37d4f9cbc3f5/test/unit/utils_test.js#L10-L11), some missing assertions were not detected:

    it('exists', () => expect(utils.fileExists(__filename)));
    it('not exists', () => expect(!utils.fileExists('not_utils.js')));

The use of expect(…) as the body of an arrow function, although unusual, does not seem invalid to me and I think it would be helpful if eslint-plugin-chai-expect would detect missing assertions in such a case as well.

So here's a PR that hopefully adds just this.

[Turbo87]

thanks :)