Add tests around connection reuse.

* bzrlib/transport/__init__.py:
(ConnectedTransport._update_credentials): New method for
transports that can authenticate again while keeping the
connection alive.

* bzrlib/tests/test_transport_implementations.py:
(TransportTests.test__reuse_for,
TransportTests.test_reuse_connection_for_various_paths): More
tests for _reuse_for.

* bzrlib/tests/test_transport.py:
(TestConnectedTransport.test_connection_sharing_propagate_credentials):
Add test for _update_credentials.
(TestReusedTransports.test_reuse_same_transport): More tests.