selftests/vsock: add vsock_loopback module loading

selftests/vsock: add vsock_loopback module loading

Add vsock_loopback module loading to the loopback test so that vmtest.sh
can be used for kernels built with loopback as a module.

This is not technically a fix as kselftest expects loopback to be
built-in already (defined in selftests/vsock/config). This is useful
only for using vmtest.sh outside of kselftest.

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com>
Link: https://patch.msgid.link/20251108-vsock-selftests-fixes-and-improvements-v4-11-d5e8d6c87289@meta.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>