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).