GFCI seemed to fizzle and pop, and now works as a non-GFCI outlet - what happened?
What happened? 


Turns out, "outdoor leak-proof covers" don't actually work. 
The GFCI was destroyed by the weather. 
Your pressing "test" exposed the weather damage, and finished the beast off. 
The noticeable arcing after you reset was grim warning of imminent hazard.  Your healthy fear of electrical things should've kicked in; it certainly would have mine! 
You tinkered with it instead; this ultimately welded the contact closed.


You now have a GFC.  Ground Fault Circuit (absolutely nothing). 

The quick fix is to replace this GFC with an outdoor-rated GFCI+receptacle.  You'll be doing this again though. 

The good fix is don't leave valuable electronics outside.  Follow this circuit back inside the building, and find the next receptacle on this circuit that's closer to the supply.  Follow the circuit from receptacle to receptacle until you find one that's in the nice dry indoors.  Fit a GFCI+receptacle there, test it, then feed the rest of the circuit off its LOAD terminals.  Replace this GFC with a plain receptacle.