Prepare test framework for pyftpdlib injection.
* bzrlib/tests/ftp_server/__init__.py:
Provide a single front-end for ftp test servers.
* bzrlib/tests/test_ftp_transport.py:
Use ftp_server import.
* bzrlib/transport/ftp/__init__.py:
(get_test_permutations): Simplified.
* bzrlib/tests/__init__.py:
(FTPServerFeature): Moved to bzrlib/tests/ftp_server/__init__.py.