test for consistent hash

I was working on switching to browserify v5 and it changed some internal APIs so the method for doing the second bundle to get a consistent hash wasn't working.

So while I was trying to figure out another way to do it, I added a test. And I couldn't get it to fail. So I reverted everything and kept the test, and even if I removed the second bundle the test always passed. The browserify bundle was the exact same, even when running 50x. Previously when @nlf and I were testing there was always a hash difference when running that many times.

I also tested against the same codebase as I did in #11 originally and after 100x the hash was always the same.

Anyway, if anyone can break this test, that'd be great :smile: But getting this merged would make updating to browserify v5 much easier.