Code review comment for lp:~vila/bzr/move-test-servers

This patch moves test servers out of the bzrlib.transport hierarchy
to bzrlib.test.test_server and all related fallouts.

It spun off bug #516183 (testools required for sftp use) to make
sure we don't run into similar problems with other transports.

It's quite big but mostly mechanical.

Given that I relied on the test suite, I don't expect a lot of fallouts.

Some plugins may break if they relied on bzrlib.transport.Server but the fix
will be trivial anyway.

They should be fine if they use our ftp or http servers though.

This patch may be easier to review commit by commit rather than as a whole...