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

[gpshead]

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 Ubuntu 24.04.1 running 6.8.0-52-generic.

• Issue: test_socket: testStream() fails on Fedora Rawhide #119461