gh-119461: Restore the testSocket VSOCK skipUnless removed by PR #119465 (#129561)

Restore the skipUnless removed by #119465.

This test can only pass on virtual machines, not actual machines.

actual machines see:

```
    self.cli.connect((cid, VSOCKPORT))
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
OSError: [Errno 19] No such device
```

Reproduced on (Linux) Ubuntu 24.04.1 running 6.8.0-52-generic.