Tests are not transpiled

[glebec]

Eventually the standard Node version will contain enough ES6 so that this becomes moot, but at least for now, FSG transpiles server & browser code but not test code. This can cause problems e.g. on Codeship if people are using the (current) default Node env of v5. A few possible solutions, one of which is already in place:

• Make sure the Codeship testing wiki instructions include nvm use v6 or similar. This is currently in effect, but is annoying to hard-code into the wiki — by rights this line probably shouldn't exist.
• Make sure the generated package.json lists a node version in engines that is ES6 compatible, as Codeship reads that file when starting a build. If we transpile tests, this isn't necessary.
• Transpile tests. This maintains parity with server & browser code and lets us easily update the transpiler for the entire project in the future, e.g. ES7 -> ES6 or similar. This is probably the best route though it is not mutually exclusive with the other options.