tests/slapgrid: make test more deterministic

test_one_failing_daemon_in_service_will_bang_with_watchdog often fail with:

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FAIL: test_one_failing_daemon_in_service_will_bang_with_watchdog (slapos.tests.slapgrid.TestSlapgridCPWithMasterWatchdog)
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Traceback (most recent call last):
  File "/srv/slapgrid/slappart9/srv/testnode/bpy/inst/test0-0/parts/slapos.core/slapos/tests/slapgrid.py", line 907, in test_one_failing_daemon_in_service_will_bang_with_watchdog
    'etc', 'software_release', 'worked', '.slapos-retention-lock-delay'])
  File "/srv/slapgrid/slappart9/srv/testnode/bpy/soft/5082e1741ad09c0910ec59bf9feae300/eggs/six-1.11.0-py2.7.egg/six.py", line 673, in assertCountEqual
    return getattr(self, _assertCountEqual)(*args, **kwargs)
AssertionError: Element counts were not equal:
First has 1, Second has 0:  'crashed'
First has 1, Second has 0:  'launched'

This test uses a service that will create files launched and crashed files when running, and just after telling supervisor to start the service test inspects the directory content. If service had time to start, then files are created, otherwise they are not.

Change the service to wait for a delay before creating the files to reduce the change of race condition here.