cover open... In fact almost closed, just to be able to take off the hdd and put it back easily.
When the computer shutdown like that it could be a heat problem or eventually a bad PSU...
You tell me that the machine have the cover open, can you visually check that all the coolers are working freely without any fuzz on them.
Make sure you check all the coolers CPU, Case, PSU and graphic card, some of them can be missing in your machine.
There is no abnormal heat, for me it is the test that has caused the shutdown, it had never occurred like that before!
I will let the pc open this night and will tell you tomorrow morning if it is still open. Ok ?