Suggested function: TestAsync

This utility function allows you to test asynchronous functions (like xhr) more seamlessly:

``` go
func TestAsync(s string, f func(QUnitAssert)) {
    Test(s, func(qa QUnitAssert) {
        done := qa.Async()
        go func() {
            defer done()
            f(qa)
        }()
    })
}
```
