Import Net::SSH::Test::Channel code from net-sftp:

This class is included in net-ssh, but net-sftp adds some new methods
that we need for our tests. This code resides in net-sftp test helper,
and can't be required dynamicaly, this change adds a copy of the code we
need, hardcoded in NetSSHStoryHelpers.

spec/support/net_ssh_story_helpers.rb

@@ -2,6 +2,24 @@ module NetSSHStoryHelpers
   require 'net/ssh/test'
   include Net::SSH::Test
 
+  # FIXME: must be moved elsewhere or implemented another way/form.
+  class ::Net::SSH::Test::Channel
+    def gets_packet(type, *args)
+      gets_data(sftp_packet(type, *args))
+    end
+
+    def sends_packet(type, *args)
+      sends_data(sftp_packet(type, *args))
+    end
+
+    private
+
+    def sftp_packet(type, *args)
+      data = Net::SSH::Buffer.from(*args)
+      Net::SSH::Buffer.from(:long, data.length + 1, :byte, type, :raw, data).to_s
+    end
+  end
+
   def story_with_new_channel
     story do |session|
       ch = session.opens_channel