Add test directory to npmignore.

The test directory consumes 3.5MB. While that in itself is not very much, many modules depend on node-multiparty. More notably connect, which in turn has over 700 dependents. All in all, it results in a lot of bandwidth and storage consumption which I think is unnecessary.

If people want to run tests code, they can clone the repo. There's no need for it to be published in npm, I think.