gpython: tests: Adjust test_pymain_syspath to current PyPy state

See https://foss.heptapod.net/pypy/pypy/-/issues/3610 for stdin case.
-m started to behave as CPython.
4 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
failed Pygolang.UnitTest-Master.Python2

00:01:41

passed Pygolang.UnitTest-Master.Python3

02:52:04

passed Pygolang.UnitTest-Master.Python2

00:02:04

passed Pygolang.UnitTest-Master.Python2

00:01:34