tests involving stub_sftp are broken on win32

All tests which make use of bzrlib.tests.stub_sftp fail on win32 with an error similar to the following:

Traceback (most recent call last):
  File "C:\Python26\lib\site-packages\testtools\runtest.py", line 129, in _run_user
    return fn(*args)
  File "C:\Python26\lib\site-packages\testtools\testcase.py", line 370, in _run_setup
    self.setUp()
  File "C:\dev\bzr\bzr.dev\bzrlib\tests\transport_util.py", line 113, in setUp
    url = self.get_url()
  File "C:\dev\bzr\bzr.dev\bzrlib\tests\transport_util.py", line 121, in get_url
    url = super_self.get_url(relpath)
  File "C:\dev\bzr\bzr.dev\bzrlib\tests\__init__.py", line 2264, in get_url
    base = self.get_server().get_url()
  File "C:\dev\bzr\bzr.dev\bzrlib\tests\__init__.py", line 2232, in get_server
    self.start_server(self.__server, self.get_vfs_only_server())
  File "C:\dev\bzr\bzr.dev\bzrlib\tests\__init__.py", line 1026, in start_server
    transport_server.start_server(backing_server)
  File "C:\dev\bzr\bzr.dev\bzrlib\tests\stub_sftp.py", line 447, in start_server
    self._homedir = getcwd()
NameError: global name 'getcwd' is not defined