modify grunt tasks inside registerTask

grunt-modify-a-task.js

    // Reconfigure tasks
    Array.prototype.map.call([
      // Unit tests should run in the background
      ['karma.unit.background', true],
      // Unit tests should run on every JS change
      ['karma.unit.singleRun', false]
    ], function(args) {
      return grunt.config.set(args[0], args[1])
    });