Add test for async functions

[cjihrig]

This test works, but currently fails linting (the parser fails). Also have to deal with Node version issues.

it('validates correct type for async functions', (done) => {

    let exception = false;
    try {
        Code.expect(async function () {}).to.be.a.function();
    }
    catch (err) {
        exception = err;
    }

    Hoek.assert(!exception, exception);
    done();
});

[cjihrig]

Adding this to the 5.0.0 milestone because lab has been updated.

[cjihrig]

Closed in bb37cc5.