Unlike -disablesafemode, all -testsafemode seems to do is display the following message in QT:

However @achow101 and @practicalswift point out there's more going on:
-testsafemodeis not unused. It is part of GetWarnings and will set a warning for strStatusBar, strRPC, and strGUI which will then trigger the safe mode handling of RPCs. Safe mode is still used in the RPCs to disable certain RPCs if safe mode is enabled.
Ah, sorry. I missed that the safe mode logic in OnRPCPreCommand is triggered by strRPC != nullptr (via GetWarnings("rpc") :-) Thanks for clarifying. Reverting that part of this PR.
That's way too indirect / convoluted for my taste. I'll just leave this PR for future reference.
PS #18 might be closable.