selftests/vsock: add tests for namespace deletion

selftests/vsock: add tests for namespace deletion

Add tests that validate vsock sockets are resilient to deleting
namespaces. The vsock sockets should still function normally.

The function check_ns_delete_doesnt_break_connection() is added to
re-use the step-by-step logic of 1) setup connections, 2) delete ns,
3) check that the connections are still ok.

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com>
Link: https://patch.msgid.link/20260121-vsock-vmtest-v16-12-2859a7512097@meta.com
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>