Add integration tests

[skx]

iz-cpm made some interesting recent commits, to add support for expect-like integration tests.

 Step::Expect("A>"),
        Step::Input("B:\r"),
        Step::Expect("B>"),
        Step::Input(&format!("ret {}\r", mode)),
        Step::Expect("B>"),

That's pretty awesome to see. Right now I can't do anything like that, stepping and getting/setting the input/output. However I do have support for pluggable output drivers.

• Create a new driver for storing characters to be output, instead of displaying them.
  • Make sure that implements a "get history" interface so we can retrieve it.
• Update our input-stuffing to cover more than just "ReadLine".

That should allow us to write a test, although the input will need to be "complete", for example:

• C:\nDIR\nEXIT

I think that should work without any single-stepping required.