support options.mockFile on coffee.fork

[fengmk2]

Usage

return coffee.fork('cli.js', args)
  .beforeScript('mock-os.js')
  .expect('stdout', 'homedir = /some/home/dir')
  .expect(code, 0)
  .end()

• mock-os.js

const mm = require('mm');
const os = require('os');

mm(os, 'homedir', () => '/some/home/dir');

• cli.js

console.log('homedir = %s', require('os').homedir());