Arbitrary preprocessor order differs between platforms

[pimterry]

Expected behaviour

• When a set of preprocessors is defined, the order may be inferred or chosen arbitrarily ('Order of Execution': http://karma-runner.github.io/1.0/config/preprocessors.html), but the same order is always used

(More generally: for a defined test suite, it should be run in the same way everywhere)

Actual behaviour

• On Linux a different arbitrary preprocessor order is used to OSX & Windows

Environment Details

• Karma version (output of karma --version): 1.3.0
• Relevant part of your karma.config.js file

{
    preprocessors: {
        '**/*.coffee': [ 'browserify' ],
        '**/*.spec.coffee': [ 'env' ]
    }
}

(Trimmed, for a full example check out this commit, which uses the config in karma.conf.coffee, which combines custom options with defaults from resin-config-karma)

On Linux this worked correctly, on OSX and Windows the resulting code didn't contain the __env__ var that karma-env-preprocessor adds. It seems that this happens if env is run before browserify, but everything works fine if they run in the other order.

Changing the second preprocessor line to '**/*.spec.coffee': [ 'browserify', 'env' ] solves the problem, but wasn't clear initially (it's easy to make this change, merge it, and not realise it won't work on other platforms until another developer not using Linux finds it and has to debug the issues).

Steps to reproduce the behaviour

Full repro:

1. Checkout this commit and npm install
2. Create a .env including the vars from the readme: https://github.com/resin-io/resin-sdk#tests
3. npm run build && npm run test-browser
4. On linux the build (e.g. our Travis) will succeed, on OSX/Windows it rapidly fails.