I could be wrong but I believe the option to destroy objects with testingcheats was patched out at some point. If you're on PC and don't mind mods there's a solution.
With MCCC open settings by clicking on a computer or by opening cheats and typing in mc_settings.
MCCC Settings > Notification/Console/Menu Settings > Console Command Settings... > Debug Commands In Cheats > Enabled
:warning: quit and relaunch the game... now shift+left-click to Destory Object (Debug)
Another option is grabbing the
Destroy Object Cheat + All Cheats mods by TwistedMexi.
