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.
See https://foss.heptapod.net/pypy/pypy/-/issues/3610 for stdin case. -m started to behave as CPython.