Modify Remote::FS interface:

* Accept SFTP session in FS constructor;
* Modify Remote to build the session required by FS;
* Change TestEnvHelpers#build_remote in order to provided a test double
  as the remote FS.

lib/producer/core/remote.rb

@@ -20,7 +20,7 @@ module Producer
       end
 
       def fs
-        @fs ||= Remote::FS.new(self)
+        @fs ||= Remote::FS.new(session.sftp.connect)
       end
 
       def execute(command)