But it seems to be pretty unsolvable in the current form; the problem is
that when you're nesting one jwack inside the other and the jobserver is GNU
make, there's no way to tell the parent jwack not to use up a token. Thus,
if you nest too deeply, it just deadlocks.
So this approach isn't really going to work the way it is.
And move test scripts into t/ subdir to keep things clean.
As part of that, make sure redo adds itself to the PATH on startup so that
subscripts can find it.
