I have a bunch of spare boards that I use for testing, from a soldered-on 386SX to PCIe. Most of them get requisitioned for testing RAM, since you kinda need the target platform to do it right, or at all.
For ISA cards, it's usually the HM386-SX board because it's small and has a friendly layout. But honestly, I don't pre-test a lot of ISA cards. I'll usually just install them in whatever system I plan to use them on. Maybe not the most cautious approach, but I guess I see the likelihood of catastrophic failure as around the noise level of general hardware failure anyway.
I will also use that board a lot for testing floppy drives, ZIP drives, things like that.
For PCI, again, it usually just goes wherever it was going to go anyway. I have a big old Gateway tower with a P3 Celeron on a random Lucky Goldstar motherboard, with a recent Gentoo Linux build on the drive. So that would be useful if/when I needed something to test on.
That Gateway used to be my hard disk testing box too, but I moved that chore to a VIA Epia build in a big Lian Li full tower with removable drive bays and 360K / 720K floppy drives for imaging to/from floppy disks. It also has a 250MB ZIP drive and SD card reader. It's basically my conduit between new skool and old skool and really old skool.