In nearly all cases, we can just switch ioutil.TempDir->testhook.TempDir
and similarly for TempFile. There's one case where we can't because we
need files to be removed before tests are over.
Also in the process give identifiable names to a lot of temporary files
and make sure they're being cleaned up, and don't use "/tmp/foo" as a
file name in a test that could be running in more than one test process
at once. :)
