So, the question is whether to interrupt an engine fire checklist and start testing fire loops
I am fairly sure no one here is suggesting interrupting memory items, rather after the engine is secured and return to earth plans are made would the test procedure be run.
Here's the thing about that: Pressing the test button may not actually reveal a bad loop as it is only a test of the detection and alarm system, post-loop, not that there's anything wrong with confirming that part of the system. I also wouldn't mind a set of MK I eyeballs looking at the thing from a window.
Apparently, the loops are under regular monitoring by the engine fire detection system - at least on our aircraft. This could be the same situation across the fleet.
Learnt a new thing yesterday, I did. From my perspective, it pays to talk to the gents in the maintenance bay.