At the college I work we've made it a point to have 1 spare system for each lab that can be used both for testing and in the event a system goes down to replace it with. While under warranty it provides minimal downtime and then when out of warranty(8yr cycle right now), well, it helps as long as not too many go down and depending on the reason. I also use VMWare Fusion for testing as well. Much of it depends on the demands of testing. My preference is working with multiple systems for some testing to test or try different combinations at once depending on what it is but if I'm going to be limited to one system and need to wipe and start over often, it's hard to beat VMware.
As far as specific hardware in recent years I've not had to worry about that other than I'm using the same hardware that's in the field(labs anyways). I just got a laptop(actually two) after all of these years so now I can do better testing for mobile use rather than going rogue and disconnecting my iMac from ethernet and connecting it to a specific wireless network. It's also tough to carry an iMac offsite if that kind of testing is needed. If testing software that might require specific video requirements like Final Cut, I'll be sure to only work on a physical system.
As for the server side of things that's where I'm at a disadvantage. Apple devices are mostly an afterthought here among the rest of the staff and my role isn't one that I have direct access to any server management. There have been discussions about a test Jamf Pro server in a test AD but the priorities of that aren't too high.
Hope that helps.
