libbinder: Check the sub-process exit status in binderRpcTest

This would have made a bug I ran into easier to diagnose (a race
condition caused the sub-process to die via a SIGPIPE. without this
change, there are no logs in that case, so it is difficult to pinpoint
where or why it died).

Test: atest binderRpcTest
Change-Id: Iaec8a6e5bb52d4e04f942418d3487f618d193236