Why test mode?
W7 & W8 require all device drivers to be signed by a trusted authority. Because CRT Emudriver is modded drivers, the signature from AMD is lost and I have to sign it myself, but I'm just a guy not a trusted authority. W7 allows drivers signed this way only if you enable a special mode called "test signing" that is meant for driver developers to test their things. W8 has made this practice even harder by requiring you to enable this test mode manually per session, making it unusable for us.
If you want a desktop system, the preference order is (leaving Linux apart):
Windows 8.1 > Windows 7 > Windows XP
If you want an emulation system, it is:
Windows XP > Windows 7 > Windows 8.1
If you only use GroovyMAME, then XP > 7 is questionable today because all fundamental issues introduced by 7 have been overcome by current builds of GroovyMAME. And W7 means real advantages in terms of hardware compatibility, with many modern mothercards not being supported by XP.
If you plan to use other emulators as well then you'll find XP more suitable. W7 introduced monitor hot plug detection that means lots of problems with arcade monitors which often are not detected. This requires adding resistors to your RGB cable to force detection. Eventhough, in W7 interlaced modes are still reported with a halved refresh, which means that D3D based emulators that attempt to vsync will run at half speed. Probably not a problem if you use a tri-sync monitor though. The fun is that running emulators other than MAME was the reason that most users employed for moving to W7 (DX11 support).