Fix check_config script

[pvizeli]

Description:

@andrey-git can you test it? I'm not sure if I mock on correct way.

Should fix this:

Exception ignored in: <bound method _UnixSelectorEventLoop.__del__ of <_UnixSelectorEventLoop running=False closed=True debug=False>>
Traceback (most recent call last):
  File "/srv/hass/lib/python3.4/site-packages/asyncio/base_events.py", line 361, in __del__
  File "/srv/hass/lib/python3.4/site-packages/asyncio/unix_events.py", line 57, in close
  File "/srv/hass/lib/python3.4/site-packages/asyncio/unix_events.py", line 138, in remove_signal_handler
TypeError: signal handler must be signal.SIG_IGN, signal.SIG_DFL, or a callable object

Related issue (if applicable): fixes #5851

[mention-bot]

@pvizeli, thanks for your PR! By analyzing the history of the files in this pull request, we identified @balloob, @kellerza and @fabaff to be potential reviewers.

[kellerza]

Who broke it? Might be better to add a unit test so that it cannot happen again

[balloob]

@kellerza you want to add a test for the test? :D

[kellerza]

But this "fix" is for bootstrap tests and a new feature in check_config

So there are no tests for the new check_config silence (or reason why not silencing it fails)

[pvizeli]

Sorry I mix two stuff in one PR. The bootstrap was not effected with check config

[balloob]

cherry-picked