gpython: tests: Skip test_pymain_run_via_relpath on SlapOS

Similarly to test_pymain_syspath we cannot run this test on SlapOS because with
buildout raw underlying python interpreter does not have access to eggs with
which gpython script was generated.

See 0fa9d6e7 and 92bb5bcc for details.

Without this patch running pygolang tests fails on SlapOS as shown below:

    (pygolang-env) slapuser34@vifibcloud-rapidspace-hosting-007:~/srv/runner/software/44fe7dd3f13ecd100894c6368a35c055/parts/pygolang-dev$ gpython -m pytest -vsx -k relpath
    ================================================= test session starts ==================================================
    platform linux2 -- Python 2.7.18, pytest-4.6.11, py-1.9.0, pluggy-0.13.1 -- /srv/slapgrid/slappart34/srv/runner/software/44fe7dd3f13ecd100894c6368a35c055/bin/gpython
    cachedir: .pytest_cache
    rootdir: /srv/slapgrid/slappart34/srv/runner/software/44fe7dd3f13ecd100894c6368a35c055/parts/pygolang-dev
    collected 115 items / 114 deselected / 1 selected

    gpython/gpython_test.py::test_pymain_run_via_relpath Traceback (most recent call last):
      File "./__init__.py", line 511, in <module>
        main()
      File "./__init__.py", line 395, in main
        pymain(argv, init)
      File "./__init__.py", line 217, in pymain
        init()
      File "./__init__.py", line 364, in init
        import gevent
    ImportError: No module named gevent
    FAILED

    ======================================================= FAILURES =======================================================
    _____________________________________________ test_pymain_run_via_relpath ______________________________________________

        @gpython_only
        def test_pymain_run_via_relpath():
            argv = ['-c',  'import sys; print(sys.version)']
            out1 = pyout(                    argv, pyexe=sys.executable)
    >       out2 = pyout(['./__init__.py'] + argv, pyexe=sys._gpy_underlying_executable, cwd=here)

    gpython/gpython_test.py:301:
    _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
    golang/golang_test.py:1842: in pyout
        return pyrun(argv, stdin=stdin, stdout=stdout, stderr=stderr, **kw)
    _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

    argv = ['./__init__.py', '-c', 'import sys; print(sys.version)'], stdin = None, stdout = '', stderr = None
    kw = {'cwd': '/srv/slapgrid/slappart34/srv/runner/software/44fe7dd3f13ecd100894c6368a35c055/parts/pygolang-dev/gpython', 'pyexe': '/srv/slapgrid/slappart34/srv//runner//shared/python2.7/44578f5389723084b136574c34bc8023/bin/python2.7'}
    retcode = 1

        def pyrun(argv, stdin=None, stdout=None, stderr=None, **kw):
            retcode, stdout, stderr = _pyrun(argv, stdin=stdin, stdout=stdout, stderr=stderr, **kw)
            if retcode:
    >           raise RuntimeError(' '.join(argv) + '\n' + (stderr and str(stderr) or '(failed)'))
    E           RuntimeError: ./__init__.py -c import sys; print(sys.version)
    E           (failed)

    golang/golang_test.py:1836: RuntimeError
    ======================================= 1 failed, 114 deselected in 0.73 seconds =======================================

Fixes 076cdd8f (gpython: Fix crash when invoked as e.g. ./bin/gpython).

/reviewed-on !11