Adding a sftptest similer to httptest

[andres-lowrie]

Hello

So I'm using this package for a project and was in the process of writing unit tests for my usage of it.

During this, I wondered if it would be worth having a package similar to https://golang.org/pkg/net/http/httptest/ ; which makes writing unit tests very easy (without having to create a lot of interfaces and what not).

Interested in knowing If adding something similar here falls into the spirt and/or strategy of this package.

Thank you.