Use mysqlhotcopy and just copy the files back.
It's a testing server. We know nothing's happening except the tests. So (for MyISAM) we can just copy the files and flush tables.
Using a mysqldump style restore was taking ~40 secs.
Copying files is taking ~1.5 seconds. And I suspect most of that is the time spent by phing and to establish an ssh connection - the actual copying directly on the server takes no time at all.
This won't work like this for InnoDB (when we get to that point). I'll need, I think, either to work out a filesystem (LVM) snapshot approach, or use Percona Xtrabackup.