I've used Windows Defender or Microsoft Security Essentials on their own for ages and never had problems. I also nowadays run them alongside a paid copy of Malwarebytes Anti-Malware which doesn't find any problems, either.
I also recommend using AdBlock with your web browsers. That might well help a few related issues down the line, especially if you are new to computers, you hate pop-up windows, or you click on everything you see!
I recommend a paid security suite especially for newcomers to computers. My favourite paid security suite to have is Norton Security. I like the way it looks and feels, and have done for years in its different guises. I also run that alongside Malwarebytes Anti-Malware. Nowadays Norton runs super smooth.
Microsoft simply includes Defender as `your first line of defense'. They don't sell it. It's free and included with Windows 8 - 10. They don't suggest not getting or replacing it with anything else.
These tests are interesting and useful I suppose. Although they're a little like taking an umbrella into a hurricane and complaining that the umbrella didn't work properly.
As a PC addict, Windows enthusiast and someone who is grateful and excited about Microsoft's role in our past, present and future, I find misleading titles to be a little wanting nowadays. I know we are all passionate about PCs and how we like to do our own things our way. But for newcomers - let's help them get the real picture shall we..