Add test case for very nested promises

[ForbesLindesay]

When attempting to fix a memory leak caused by nested promises I inadvertently completely broke my library once more than a couple of promises were nested (see then/promise#67). I caught it with the following test:

    var promises = [];
    var resolveFns = [];
    for (var i = 0; i < 100; i++) {
      promises.push(new Promise(function (resolve) {
        resolveFns.push(resolve);
      }));
    }
    for (var i = 0; i < 99; i++) {
      resolveFns[i](promises[i + 1]);
    }
    resolveFns[99]('foo');
    return promises[0].then(function (result) {
      assert(result === 'foo');
    });

It took me a long time to realise that I might have an bug in that place so it would be great if we could add a test like this so that other people don't suffer the same problem.

[domenic]

For sure, PR welcome. Should be pretty easy.