ipv4: fix a race in ipv4_method_shared_with_already_running_dnsmasq

dnsmasq instance we start with --bind-interfaces would bind to all interfaces with an address on startup. The test expects it to bind before NetworkManager-managed instance of dnsmasq attempts to bind and end up failing. Given it's started in background when the interface is not configured yet, it may or may not end up trying to bind at the right time.

It inded fails occassionally, roughly once in a few hundred runs.

This patch makes sure the interface is configured at the time dnsmasq is started and gives it hopefully enough time to bind.