Fixes and add smoke test: hard timeout on isolated task.

client/utils/subprocess42.py

Index: client/utils/subprocess42.py
diff --git a/client/utils/subprocess42.py b/client/utils/subprocess42.py
index ef82dcdf70e49d7087f27d20b1456ddca99c8ce8..ad56bdb010996b76af66b53c7936463723b506b1 100644
--- a/client/utils/subprocess42.py
+++ b/client/utils/subprocess42.py
@@ -454,8 +454,11 @@ def set_signal_handler(signals, handler):
 @contextlib.contextmanager
 def Popen_with_handler(args, **kwargs):
   proc = None
-  def handler(_signum, _frame):
+  def handler(signum, _frame):
     if proc:
+      logging.info('Received signal %d; terminating', signum)
+      # There could be a race condition where the process already exited. Our
+      # subprocess implementation traps this.
       proc.terminate()
 
   with set_signal_handler(STOP_SIGNALS, handler):