Develop set of tests that emulate Webmaker App's use of MakeDrive

[humphd]

Our first production use case for MakeDrive is Webmaker App, and I'd like to get a regression test suite that covers all the various ways that it uses MakeDrive. At the moment, this includes things like:

• fs.readFile and fs.writeFile for UTF8 JSON files
• fs.stat
• fs.unlink
• sh.rm
• sync events

Most of the code that uses MakeDrive is in the following files:

• https://github.com/mozillafordevelopment/webmaker-app/blob/eccf869fd8db7b5a24072d679508a24f5483e705/lib/model.js
• https://github.com/mozillafordevelopment/webmaker-app/blob/eccf869fd8db7b5a24072d679508a24f5483e705/views/share/index.js

I'd like to see these tests be easily runnable from a browser, since I want to use this as a way to test MakeDrive on various devices/OSes/browsers.