Apply test_connection_sharing to all transports to make smart
transports fail the test.

* bzrlib/tests/test_transport_implementations.py:
(TransportTests.test_connection_sharing): Moved and updated from
test_transport.py.

* bzrlib/tests/test_transport.py:
(TestConnectedTransport.test_connection_sharing): Moved to
test_transport_implementations.py.