This happened to me when I tried out Trackpad++ which is an app that supposedly provides OSX Safari gestures in Windows 10 via Bootcamp 6. After installing Trackpad++ I got the 'test mode' watermark which I could not stand.
Simply removing Trackpad++ and its brutal resident program, Power Plan Assistant, all I needed to do was re-run the Boot Camp 6 install (choosing Repair) and run a simple elevated CMD prompt (below) and it go things back to normal, no more test mode watermark.
The elevated CMD prompt is: bcdedit -set TESTSIGNING off
If you didn't mess with Trackpad++, try reinstalling Boot Camp 6 ("Repair") and running that CMD prompt and reboot.