Alternatively they could test their shit in advance

A big part of the problem was 3rd party programs that was not ready. A big change was introduction of the User Account Control (UAC) that more or less started to force programs to behave better: Install into program file, save stuff in user profile, don't do dumb admin stuff if not needed, making programs start to behave more like they lived in a multi-user operating system. It was a change that had to be done and it was never going to be a good experience.

It’s not like Microsoft is too poor to afford an array of average computers and a dozen of testers

I think you underestimate how many testers and how much work actually goes into testing both Microsoft's own software and work with 3rd party software vendors to make sure their software worked. This has changed somewhat, with Windows 10 and forward, where you have a lot more beta testing in the public.

I agree that there should have been spent more time on testing Vista and given more time to 3rd party to test their stuff. However, 3rd party software and drivers took, in some instances, 1-2 years after Vista release, before they updated their stuff to work with Vista. There were just not a lot of companies interested in spending the money and time to make make it work as Vista got a (deserving) bad reception, but a big part of the problem was these companies. A chicken/egg situation.