[rfc] initial conformance test for programs/cmd

[Dan Kegel]

How's this look?  I've only tested it lightly, but it should
be pretty close; with this, "make test" ought to recurse
down into programs/cmd/tests and run the cmd tests, too.

Shortcomings:
- I don't think I need chomp() in tests/cmd.c anymore, that was a workaround
- I think I can turn on echo again in tests/*.cmd, that was another workaround
- I have yet to add a way to specify that a test is todo_wine

Feedback welcome.  I'll try to submit something tomorrow
that takes care of the above known issues and any issues
anyone else points out.

(I'm actually kind of amazed the build machinery changes work,
I didn't really take the time to understand all of the gears and pulleys.)