INode.get_output allows receiving stdout of remotely run commands (fixes 222)

Fixes #222.
This style of testing which puts the assumption about `b"hello"` being the result in a different place than it puts the test fixture which is responsible for producing this output seems sub-optimal to me.  The resulting tests seem more fragile than necessary.  At the very least, the requirement that running `[b"-n", b"hello"]` (a somewhat nonsensical thing to run) produce the output `b"hello"` (but once and only once!) should be documented on `make_inode_tests`.

Making it possible to repeat a command on `FakeNode` would simplify this a bit.  Making `FakeNode` a little more capable would probably help even more.  Since the interface is so wide ("do anything you can do with a shell") it's pretty hard to make `FakeNode` even remotely close to as capable as the real implementation though.  I think this probably means `INode` is a bad interface.  But we knew that already.

I suppose the thing to do is fix the documentation point and hope we can replace all of this with something else soon.