Fix multiple attach test.

This increases the window between the sleep and the attach; this allows the `docker run` to finish processing even on the slowest systems (like my VM!).

I've run the test about 20 times without a single issue. in `#docci-test` on freenode you can see this test failing about 50% of the time at the moment.
You can write:
```
case <-time.After(15 * time.Second)
```
and this will be better because you see actual channel meaning where you use it.