vsock/test: Introduce vsock_bind()

Add a helper for socket()+bind(). Adapt callers.

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Luigi Leonardi <leonardi@redhat.com>
Signed-off-by: Michal Luczaj <mhal@rbox.co>
Link: https://patch.msgid.link/20250128-vsock-transport-vs-autobind-v3-3-1cf57065b770@rbox.co


Signed-off-by: Jakub Kicinski <kuba@kernel.org>