ssh: make godoc examples easier to reuse

Fixes golang/go#9747

Move the example tests to an external test package so that they
must explicitly reference the ssh package. The side effect is the
examples now become easier to copy and paste.

Change-Id: Ibbddea42bc5a41d11ffdef5144d9884ef3ef603f
Reviewed-on: https://go-review.googlesource.com/3710
Reviewed-by: Andrew Gerrand <adg@golang.org>
2 files changed