Wrap tests in domain instead of try/catch

In several cases, async errors with nodeunit and streams2 only result in the test giving a "test not run or test.done not called". without any sign of the error. What I ended up doing is something like this

``` javascript
Function.prototype.withDomain = function(withStack) {
var fn = this;
return function(test) {
var d = domain.create();
d.on('error', function(e) {
test.fail('test failed with ' + e.message);
if(withStack) {
console.error(e.stack)
}
test.done();
});
d.run(fn.bind(this, test));
}
}

exports['my test'] = function (test) {
setTimeout(function() {
throw new Error('Throwing an error asynchronously');
test.done()
});
}.withDomain()

```

I just thought this was useful enough to be the default.