You're constructing an argument that's so specific, you're basically begging to be the victim.
If you a) have a device that can't make regular calls b) have no way to contact a non-emergency number through a different device to ask if it's okay to make a test call and c) absolutely need to test this right now, then you're about one in a million. Just make that call, answer the "what's your emergency?" question with "No emergency right now, I need to test that my device that can't make regular calls will work in a real emergency. Sorry, I couldn't reach your non-emergency number." and it will be fine.
It's only an illegit 911 call if you call them for fun or if you test so often it can no longer be considered necessary.