RFR(s): 8157777: RMI test DeadCachedConnection doesn't wait for registry to die

[Stuart Marks]

Hi all,

Please review this small test fix to improve the reliability of an RMI test. 
Basically this waits for a subprocess to exit instead of proceeding immediately.

webrev:

     http://cr.openjdk.java.net/~smarks/reviews/8157777/webrev.0/

bug:

     https://bugs.openjdk.java.net/browse/JDK-8157777

Thanks,

s'marks