Fix integration tests

Follow up for https://github.com/containerd/rust-extensions/issues/110
There are a couple of integration tests that we disabled on CI (https://github.com/containerd/rust-extensions/pull/104) which require investigation:
- `TestContainerPTY`
- `TestContainerExecLargeOutputWithTTY`
- `TestTaskUpdate`
- `TestTaskResize`

We should fix the shim and enable them back.