[MERGE] More than double the speed of "bzr selftest"

[Martin Pool]

I was hoping to do this while Andrew was away.  I didn't finish it and
am going away myself for a couple of weeks.  Anyhow, here it is:

https://code.launchpad.net/~mbp/bzr/test-cleanup

I started from scratch to cleanup several things in how test
directories are handled.  I'd like to get rid of --keep-output and
remove some accumulated cruft too.

-- 
Martin