Resolve memory leaks in NSS unit tests
These leaks seem to be caused by the way that the unit tests fork and reinitialize NSS: they don't appear to affect Tor itself. That said, we'll need to get rid of them, to make sure that we're able to find real memory leaks.
One possibility here is to refactor our test harness so that NSS initialization happens only in subprocesses, and so that forking always happens. I'm not sure I like that, but most other things I have tried seem to fail.
Tor itself doesn't have these leaks.