cmd: add support for todo_wine constructions in testing infrastructure

[Dan Kegel]

I agree @todo_space@ should go; it's never used, so it's dead code now.
Tempting to say that removing it should be a separate patch,
rather than combining that with adding a new feature.
Since all the @keywords@ are poorly documented,
I'm glad to see an unused one go.

Your generic @todo_wine@ keyword seems useful.

The current code is a bit ugly.  If you're going to add a
nice is_todo_wine_line() helper, it might make sense
to do a cleanup patch that cleans up the @keyword@
recognition in general first.  (Why the verbose array
declaration instead of a "constant"?  etc.)

I see you're making every line of the .exp file count as
a test.  That seems ok.

There's a fair bit of code duplication; I wonder if you
could avoid that by refactoring things a bit.

To avoid adding dead code, it might be nice to
see a test that actually uses your new keyword.
- Dan