Add browserifytest to Cakefile.

This adds the failing browserify test @pwnall requested. How it works:

1. Run `cake browserifytest` on this branch. The test passes!
2. Revert c2b797baaa65280d2a59c7086b339f63a9cc842d, which fixed browserify support.
3. Run `cake browserifytest` again. The test fails!