Hi
This won't be possible unless you move the BD and FW-L3out to an other VRF. If possible try to do your testing the other way around: route a specific destination (some kind of external test-network) via the FW-L3out, all your BDs/EPGs will use the FW as a next-hop if you point a more specific route to the FW.
It's also possible to achieve this using policy based redirect: you need to attach the FW via a service BD (not via a l3out), you can then attach a contract with a service graph between your test BD/EPG and your core-l3out external EPG and redirect the corresponding traffic to the FW.