Separate transport from test server.

* bzrlib/transport/ftp.py:
Move ftp test server to its own file.
(get_test_permutations): Reworked around FTPServerFeature.

* bzrlib/tests/test_ftp_transport.py:
(TestCaseWithFTPServer.setUp): Use FTPServerFeature instead of
MedusaFeature.

* bzrlib/tests/__init__.py:
(_FTPServerFeature): New feature allowing a cleaner separation
between ftp.py and FTPServer.py.

* bzrlib/tests/FTPServer.py: 
New file. Extracted from bzrlib/transport/ftp.py (use case for
tracking moving lines).