golang: tests: Fix $PYTHONPATH handling in _pyrun

We were appending whole $PYTHONPATH to pathv - a vector of path.
But we should instead first split $PYTHONPATH by pathsep and extend pathv.

golang/golang_test.py

@@ -1821,7 +1821,7 @@ def _pyrun(argv, stdin=None, stdout=None, stderr=None, **kw):   # -> retcode, st
     env.update(envadj)
     envpath = env.get('PYTHONPATH')
     if envpath is not None:
-        pathv.append(envpath)
+        pathv.extend(envpath.split(os.pathsep))
     env['PYTHONPATH'] = os.pathsep.join(pathv)
 
     p = Popen(argv, stdin=(PIPE if stdin else None), stdout=stdout, stderr=stderr, env=env, **kw)