Martin,
There is no mraid.js file to download that I know of. This is normally integrated into the SDK but is only a facade to connect to the native layer.
I suspect that creating a mock-object would be fairly simple, but maybe not necessary. We regularly use the web tester instead and that allows us to see how the ad reacts to the confined container.
As far as a place to point to your URL, I would give the webtester a try. The flight tab includes a field to accept a URL, but there are some caveats. Some browsers simply don't allow for cross-domain content -- but you can usually override those settings. The other option is to download the code from Google Code and run the webtester locally.
Hope that helps.
Thanks,
-Nathan