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.
25 jobs for fix/pygolang-S in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-Jerome.Python2

00:23:07

passed SlapOS.Eggs.UnitTest-Jerome.Python3

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passed SlapOS.Eggs.UnitTest-Jerome.Python2

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failed SlapOS.Eggs.UnitTest-Jerome.Python2

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passed SlapOS.Eggs.UnitTest-Jerome.Python2

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passed SlapOS.Eggs.UnitTest-Jerome.Python2

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passed SlapOS.Eggs.UnitTest-Jerome.Python2

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passed SlapOS.Eggs.UnitTest-Jerome.Python2

passed SlapOS.Eggs.UnitTest-Jerome.Python2

00:37:55

passed SlapOS.Eggs.UnitTest-Jerome.Python2

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failed SlapOS.Eggs.UnitTest-Jerome.Python2

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passed SlapOS.Eggs.UnitTest-Jerome.Python2

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passed SlapOS.Eggs.UnitTest-Jerome.Python2

00:15:50

passed SlapOS.Eggs.UnitTest-Jerome.Python2

00:07:05

passed SlapOS.Eggs.UnitTest-Jerome.Python2

passed SlapOS.Eggs.UnitTest-Jerome.Python3

00:16:22

passed SlapOS.Eggs.UnitTest-Jerome.Python3

00:23:14

passed SlapOS.Eggs.UnitTest-Jerome.Python3

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passed SlapOS.Eggs.UnitTest-Jerome.Python3

00:13:50

failed SlapOS.Eggs.UnitTest-Jerome.Python3

00:25:24

failed SlapOS.Eggs.UnitTest-Jerome.Python3

passed SlapOS.Eggs.UnitTest-Jerome.Python3

00:16:54

passed SlapOS.Eggs.UnitTest-Jerome.Python3

passed SlapOS.Eggs.UnitTest-Jerome.Python3

00:35:58

passed SlapOS.Eggs.UnitTest-Jerome.Python3

00:30:43