Fix bechmark for promises-native-async-await.js

[Mirodil]

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Please answer the questions the best you can:

1. What version of bluebird is the issue happening on? master branch @3.7.2

2. What platform and version? (For example Node.js 0.12 or Google Chrome 32) macOS nodejs 10.22.1

3. Did this issue happen with earlier version of bluebird? None

I think the comparison in the benchmark is not fair as promises-bluebird.js uses concurrency, but promises-native-async-await.js uses not concurrency. Here is a code example:

// file promises-bluebird.js 
var blobIdP = blob.put(stream);
var fileP = self.byUuidOrPath(idOrPath).get();
var version, fileId, file;

bluebird.join(blobIdP, fileP, function(blobId, fileV) { // <----this is concurrent
    ...
})

vs

var blob = blobManager.create(account);
var tx = db.begin();
var blobId = await blob.put(stream); // <----this is not concurrent
var file = await self.byUuidOrPath(idOrPath).get(); // <----this is not concurrent

At least should be the following to be fair:

var blob = blobManager.create(account);
var tx = db.begin();
var blobId = blob.put(stream);
var file = self.byUuidOrPath(idOrPath).get();
[blobId, file] = await Promise.all([blobId, file]);// <----this is concurrent