move the transform field into test/package.json

This patch fixes the issue I was getting from browserify core with [the new patch](https://github.com/substack/node-browserify/pull/721):

```
$ browserify -r zlib
Error: Cannot find module '/home/substack/projects/node-browserify/node_modules/browserify-zlib/brfs' from '/home/substack/projects/node-browserify'
```