Personally I think testing should be performed with a range of ordinary PCs as that is representative of a typical scenario. There are so many variables with PCs and even with an "optimised" one, the OS will still be doing its own thing which you won't have much control over.
I sort of think it's the wrong approach to require a "special" PC to make your dac sound good, the dac should cope with what's thrown at it. That's meant to trivialise the potential difficulties in achieving that, but I don't think it is anything that good engineering research and design can't solve.
A secondary question here is, "does my own dac (whatever that may be) significantly benefit from an optimised PC?" Well unfortunately, unless you are john and have the necessary measurement kit and knowledge to look at things such as the clock, it's phase noise etc etc then you will not objectively know. All you can do is spend money on a special PC and see if you subjectively think there's an improvement.............to me unfortunately that's a bit random, subject to the vagaries of the human condition
and potentially costly..... Hopefully the detox will render all this moot.