Fix last failing tests under python2.6.

* bzrlib/tests/ftp_server/pyftpdlib_based.py:
(FTPServer.get_url, FTPServer.setUp): Because medusa authorized
connections without checking the password, we have tests that more
or less relies on that behavior in direct or indirect ways. The
simplest solution is to give write access to anonymous with
pyftpdlib and test authentication in a separate test.

* bzrlib/tests/test_ftp_transport.py:
(TestFTPServerUI.setUp, TestFTPServerUI.get_url): Implementation
compatible with medusa and pyftplib.
(TestFTPServerUI.test_prompt_for_password,
TestFTPServerUI.test_no_prompt_for_password_when_using_auth_config):
Simplified.

* bzrlib/tests/ftp_server/medusa_based.py:
(FTPServer.add_user): Factored out from
bzrlib.tests.test_ftp_transport.TestFTPServerUI._add_authorized_user
since that's specific to our medusa based ftp server.

* bzrlib/tests/test_transport_implementations.py:
(TransportTests.test_connect_twice_is_same_content): Previous fix
was incorrect, '.base' contained the needed server prefix, using
self.get_transport instead of the basre get_transport provides the
wanted feature.