tests/test_slapgrid: don't use python -S in test scripts

I guess this code was using python -S to try to be independant from anything
that can happen in site.py, but that's not needed when running on testnode.

pygolang.pymain does not support -S and we are moving tests environments to
use pygolang.pymain, so let's remove this if we don't need it.

slapos/tests/test_slapgrid.py

@@ -70,7 +70,7 @@ import httmock
 dummylogger = logging.getLogger()
 
 
-WATCHDOG_TEMPLATE = """#!{python_path} -S
+WATCHDOG_TEMPLATE = """#!{python_path}
 import sys
 sys.path={sys_path}
 import slapos.slap