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

client/utils/subprocess42.py

 # Copyright 2013 The Swarming Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0 that
 # can be found in the LICENSE file.
 
 """subprocess42 is the answer to life the universe and everything.
 
 It has the particularity of having a Popen implementation that can yield output
 as it is produced while implementing a timeout and not requiring the use of
 worker threads.
 
@@ skipping 436 matching lines @@
   try:
     yield
   finally:
     for sig, h in previous.iteritems():
       signal.signal(sig, h)
 
 
 @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):
     proc = Popen(args, detached=True, **kwargs)
     try:
       yield proc
     finally:
       proc.kill()