verifyNoStaleFiles misses Docker containerd task path in moby namespace

[mdryaan]

Description

`verifyNoStaleFiles` in `tests/e2e/utils.go` checks containerd task paths under `default` and `k8s.io` but does not check the `moby` namespace. Docker
uses the `moby` namespace for containerd tasks. As a result, post-test stale file verification never catches leftover containerd task directories when tests
run with Docker, which defeats the purpose of the check.

Fixes #64

Current Behavior

Only two task paths are checked:

• `/run/containerd/io.containerd.runtime.v2.task/default/`
• `/run/containerd/io.containerd.runtime.v2.task/k8s.io/`

The `moby` path used by Docker is not checked. A stale directory under `/run/containerd/io.containerd.runtime.v2.task/moby/` will not be
detected and the function returns nil.

Expected Behavior

`verifyNoStaleFiles` should verify all three containerd namespaces that urunc can place task directories under: `default`, `k8s.io`, and `moby`.