i've got it!

callback-foo.js

it('complains when the p2p or location values are not valid', function(done) {
  var self = this;
  // YAY higher order programming :)
  var expectError = function(cb) {
    return function(err, result) {
      expect(err).to.be.an.instanceof(Error);
      cb(null)
    };
  };
  async.parallel([
    function(cb) {
      self.opentok.createSession({ p2p: 'lalalal' }, expectError(cb))
    },
    function(cb) {
      self.opentok.createSession({ location: 'not an ip address' }, expectError(cb))
    }
  ], done);
});