Add timeout to test_async_stream

Related to #11700 

Without timeout:

this test executed infinitely long on JRuby, as seen in the screenshot from Travis. see the 2nd row:
![](https://f.cloud.github.com/assets/235844/901707/592549e6-fb85-11e2-812a-1b9610125744.png)



With Timeout:
this test gracefully fails on JRuby, test on v1.7.4
Passes on MRI, tested on v2.0 & v1.9.3