I just had a real user in my "test users only" user overview. The test user's name contained XXx. Although this is probably a rare case, I would suggest to prohibit XXX (and its lowercase variants) in the random strings for real users to ensure that no real users are mistaken for test users.
we store whether someone is a test user as a flag and should use that more rather than this short substring. we might also make the substring longer...
I just had a real user in my "test users only" user overview. The test user's name contained XXx. Although this is probably a rare case, I would suggest to prohibit XXX (and its lowercase variants) in the random strings for real users to ensure that no real users are mistaken for test users.