I don’t know, but if you’re shipping mainboards with soldered on RAM it could be worth it to test the modules before soldering to reduce the amount of returns where you have to replace the entire board.
Dram, Flash, CPUs, GPUs and most other high-margin chips made by established manufacturers(Intel, Micron, Samsung, TI, NXP, etc.) are tested to some degree at the fab before packaging. It's very common in the semiconductor industry.
That doesn't mean parts with some defects don't sneak into the final products but the rate is low enough that testing each chip you receive is not monetary feasible for consumer products so you just test the final assembled product to a degree, but even then, brand new Apple products are no strangers to having their mainboards replaced for a defective chip in the warranty period. Just ask Luis Rossmann or people working at the genius bar. That's why everyone recommends you get Apple care.
So the theory that Apple thoroughly tests each chip that goes in their products is false. It would be too expensive and eat into their fat margins.
What you do test, is samples from each batch you receive, to make sure your supplier did not pull a bait and switch and replaced your contracted parts with lower quality ones under the same part number and hope you would not notice but this is more common on low margin parts and Chinese suppliers are notorious for this since for them it's just regular business practice and see nothing wrong with it.
We did a study on how often we used AppleCare vs the cost and, to no one’s surprise, AppleCare is not financially worthwhile on computers if the small chance of having to replace the hardware won’t be a significant hurdle. (It doesn’t cover liquid damage, which are a significant fraction of early hardware replacement.)
