Bug 800402 - [email] UI: Show the number of unread messages in the message-list header after the folder name

So, if we only need that in this function (which should be fine I think):
```js
var flushCount = 0;
ctx.storage.flushExcessBlocks = function() { flushCount++; };
```

is probably fine.  If we wanted to reuse that a lot, I'd have makeTestContext() do the clobbering and have it update ctx.flushCount or something like that.