So if we're in redo/t/ and running 'redo hello', we'll detect redo/.redo and
use that one rather than creating a new redo/t/.redo directory.
The downside of this is we get slightly different behaviour if the *first*
thing you build isn't from the root. Probably that's bad, but it should
hopefully be rare.
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.
