How to test my Site to Site VPN on a new Firewall before deploying it on site.

Hello networking Community!

You are kind of my last hope since nobody at my workplace or my Web Searches can comprehend the problem I got here.

Since I started in IT everybody always tells me ALWAYS test everything before you deploy/configure/change anything! So that's what I'm trying to live by. Here is my Layout. We have a Sophos Firewall SG 210 inhouse and a PFsense Firewall on a remote Site. There is a VPN IPSec tunnel configured between these Firewalls with each having the remote Gateway and preshared Key set. Now we want to replace the PFSense Firewall with a small Sophos Sg 105. The IP/Gateway etc. stay the same only the hardware and config are going to change. That's all good but how am I able to test the new VPN IPsec tunnel between the two Sophos Firewalls, when I only have one physical WAN/Internet Interface/connection for the Inhouse firewall and none for my new Remote Site Firewall. Is there a way that I can create a "pseudo" WAN for the new Firewall to be behind so that I can test the VPN Tunnel?

I hope I made my problem understandable.