Add half-baked test for Net::SFTP.start
user
```ruby
require 'common'

class StartTest < Net::SFTP::TestCase
  def test_with_block
    ssh = mock('ssh')
    ssh.expects(:close)
    Net::SSH.expects(:start).with('host', 'user', {}).returns(ssh)

    sftp = mock('sftp')
    # TODO: figure out how to verify a block is passed, and call it later.
    # I suspect this is hard to do properly with mocha.
    Net::SFTP::Session.expects(:new).with(ssh).returns(sftp)
    sftp.expects(:connect!).returns(sftp)
    sftp.expects(:loop)
    
    Net::SFTP.start('host', 'user') do
      # NOTE: currently not called!
    end
  end
end
```
assistant