test suite leaks fds

[William Reade]

Run "./test -u", and you'll eventually see failures due to fd exhaustion.

I've written some code in lp:~fwereade/ensemble/audit-test-fds to track what's going on slightly more closely, and it seems to me that ~500 of our ~1200 tests leak a total of ~600 fds every run.

Fixing all these will probably be quite a lot of work, but it seems probable that many of the tests are leaky because the underlying code is too. If we could be confident that our tests weren't leaking I think we could be a lot more confident in the real code too, and that would surely be a Good Thing.