tests: `pyvenv --system-site-packages` is too buggy on python 3.4

tests: `pyvenv --system-site-packages` is too buggy on python 3.4

It always wants to install packages into /usr/lib/python3.4/site-packages

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/pip/basecommand.py", line 122, in main
    status = self.run(options, args)
  File "/usr/lib/python3/dist-packages/pip/commands/install.py", line 295, in run
    requirement_set.install(install_options, global_options, root=options.root_path)
  File "/usr/lib/python3/dist-packages/pip/req.py", line 1436, in install
    requirement.install(install_options, global_options, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/pip/req.py", line 672, in install
    self.move_wheel_files(self.source_dir, root=root)
  File "/usr/lib/python3/dist-packages/pip/req.py", line 902, in move_wheel_files
    pycompile=self.pycompile,
  File "/usr/lib/python3/dist-packages/pip/wheel.py", line 214, in move_wheel_files
    clobber(source, lib_dir, True)
  File "/usr/lib/python3/dist-packages/pip/wheel.py", line 176, in clobber
    os.makedirs(dest)
  File "/usr/lib/python3.4/os.py", line 237, in makedirs
    mkdir(name, mode)
PermissionError: [Errno 13] Permission denied: '/usr/lib/python3.4/site-packages'