tests/slapgrid: make test more deterministic

`test_one_failing_daemon_in_service_will_bang_with_watchdog` often fail with:

```
======================================================================
FAIL: test_one_failing_daemon_in_service_will_bang_with_watchdog (slapos.tests.slapgrid.TestSlapgridCPWithMasterWatchdog)
----------------------------------------------------------------------
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.

/reviewed-on nexedi/slapos.core!82

slapos/tests/slapgrid.py

@@ -96,7 +96,7 @@ chmod 755 etc/run/wrapper
 DAEMON_CONTENT = """#!/bin/sh
 mkdir -p etc/service &&
 echo "#!/bin/sh" > etc/service/daemon &&
-echo "touch launched
+echo "sleep 1; touch launched
 if [ -f ./crashed ]; then
 while true; do echo Working; sleep 0.1; done
 else