Register inotify watches on all cgroup hierarchies.

We used to only register them on the first hierarchy that was created (I
think this was unintentional). This caused some weird edgecases where
we'd try to delete a watch event we didn't create. It is an error we
ignore today (since we fix it in < 60s) but delays our destruction of
the container.
Merge pull request #326 from vmarmol/integration

Register inotify watches on all cgroup hierarchies.
Update testify dependency.
Merge pull request #327 from vmarmol/deps

Update testify dependency.