Well, you really don't want to use ICMP redirects if possible.
You really want:
Test Hosts -------FW/Router -------- ISP router
This creates three networks, and your FW/Router does your LAN routing, leaving the ISP router as the internet gateway.
ICMP redirects are advisory, and can be used maliciously, so are often disabled by default.
Since you don't have a routing network where routing protocols can have much impact, and because ICMP redirects are at a much lower layer, you'll not accomplish what you want.