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/fs.rb

@@ -2,14 +2,10 @@ module Producer
   module Core
     class Remote
       class FS
-        attr_reader :remote
+        attr_reader :sftp
 
-        def initialize(remote)
-          @remote = remote
-        end
-
-        def sftp
-          @sftp ||= @remote.session.sftp.connect
+        def initialize(sftp)
+          @sftp = sftp
         end
 
         def dir?(path)