As far as I know, the actual testing is done by the vendor. It might get reviewed by an MS or Apple imployee, but that isn't the same thing as actively testing the hardware themselves. Or providing software to work with third party hardware.
It's a chicken and egg problem. Hardware vendors don't worry about linux compatibility, at least for consumer/desktop products because the market share is low. But one reason lack of good hardware support hinders adoption of linux on desktop.
And what happens if it doesn't work? MS and Apple aren't going to change their software to work with the device, they will tell the vendor to fix it. But for linux it is the other way around.
Raymond Chen's blog 'The Old New Thing' details the steps MS took to ensure that partner vendor products _would_ work with Windows. MS tested products extensively and did modify their own product (Windows) to ensure that products supplied by partner vendors worked reliably and consistently between different windows versions.
That is making sure that things that worked on an old version work on a new version. That's not the same as making sure something new works on an existing version of the OS.
It's a chicken and egg problem. Hardware vendors don't worry about linux compatibility, at least for consumer/desktop products because the market share is low. But one reason lack of good hardware support hinders adoption of linux on desktop.