Launch testing doesn't kill nodes correctly if process fails to be spawned

[dhood]

If two processes are added to a launch descriptor, and one of them launches fine but the other one causes an error, the first one keeps persisting as a zombie process.

Steps to reproduce:

Use this fork with a test that tries to spawn a talker node and a nonexistent fail node: https://github.com/dhood/launch/blob/zombie_nodes/launch_testing/test/test_zombie_nodes.py

$ ament test --isolated --only-packages launch_testing

The talker will start then the test will crash because of the nonexistent node.

======================================================================
ERROR: test_zombie_nodes.test_zombie_nodes
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/dhood/.local/lib/python3.5/site-packages/nose/case.py", line 198, in runTest
    self.test(*self.arg)
  File "/home/dhood/ros2_ws/src/ros2/launch/launch_testing/test/test_zombie_nodes.py", line 22, in test_zombie_nodes
    rc = launcher.launch()
  File "/home/dhood/ros2_ws/install_isolated/launch/lib/python3.5/site-packages/launch/launcher.py", line 96, in launch
    raise e.exception
  File "/home/dhood/ros2_ws/install_isolated/launch/lib/python3.5/site-packages/launch/launcher.py", line 127, in _run
    await self._spawn_process(index)
  File "/home/dhood/ros2_ws/install_isolated/launch/lib/python3.5/site-packages/launch/launcher.py", line 351, in _spawn_process
    **kwargs)
  File "/usr/lib/python3.5/asyncio/base_events.py", line 1079, in subprocess_exec
    bufsize, **kwargs)
  File "/usr/lib/python3.5/asyncio/unix_events.py", line 184, in _make_subprocess_transport
    **kwargs)
  File "/usr/lib/python3.5/asyncio/base_subprocess.py", line 40, in __init__
    stderr=stderr, bufsize=bufsize, **kwargs)
  File "/usr/lib/python3.5/asyncio/unix_events.py", line 649, in _start
    universal_newlines=False, bufsize=bufsize, **kwargs)
  File "/usr/lib/python3.5/subprocess.py", line 947, in __init__
    restore_signals, start_new_session)
  File "/usr/lib/python3.5/subprocess.py", line 1551, in _execute_child
    raise child_exception_type(errno_num, err_msg)
FileNotFoundError: [Errno 2] No such file or directory: 'fail'
-------------------- >> begin captured logging << --------------------
asyncio: DEBUG: Using selector: EpollSelector
--------------------- >> end captured logging << ---------------------
Exception ignored in: <bound method BaseEventLoop.__del__ of <_UnixSelectorEventLoop running=False closed=True debug=False>>
Traceback (most recent call last):
  File "/usr/lib/python3.5/asyncio/base_events.py", line 431, in __del__
  File "/usr/lib/python3.5/asyncio/unix_events.py", line 58, in close
  File "/usr/lib/python3.5/asyncio/unix_events.py", line 139, in remove_signal_handler
  File "/usr/lib/python3.5/signal.py", line 47, in signal
TypeError: signal handler must be signal.SIG_IGN, signal.SIG_DFL, or a callable object
Exception ignored in: <bound method BaseSubprocessTransport.__del__ of <_UnixSubprocessTransport closed pid=3339 running stdin=<_UnixWritePipeTransport closing fd=15 open> stdout=<_UnixReadPipeTransport fd=16 open>>>
Traceback (most recent call last):
  File "/usr/lib/python3.5/asyncio/base_subprocess.py", line 126, in __del__
  File "/usr/lib/python3.5/asyncio/base_subprocess.py", line 101, in close
  File "/usr/lib/python3.5/asyncio/unix_events.py", line 568, in close
  File "/usr/lib/python3.5/asyncio/unix_events.py", line 560, in write_eof
  File "/usr/lib/python3.5/asyncio/base_events.py", line 497, in call_soon
  File "/usr/lib/python3.5/asyncio/base_events.py", line 506, in _call_soon
  File "/usr/lib/python3.5/asyncio/base_events.py", line 334, in _check_closed
RuntimeError: Event loop is closed

Starting a listener you will notice the talker is still alive:

$ listener
I heard: [Hello World: 508]
I heard: [Hello World: 509]
I heard: [Hello World: 510]

[dhood]

with #55 the output becomes:

======================================================================
FAIL: test_zombie_nodes.test_zombie_nodes
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/dhood/.local/lib/python3.5/site-packages/nose/case.py", line 198, in runTest
    self.test(*self.arg)
  File "/home/dhood/ros2_ws/src/ros2/launch/launch_testing/test/test_zombie_nodes.py", line 24, in test_zombie_nodes
    assert rc == 0, "The launch file failed with exit code '" + str(rc) + "'. "
AssertionError: The launch file failed with exit code '1'. 
-------------------- >> begin captured logging << --------------------

and no error runtime error is printed