I have installed Fedora on VirtualBox specially for malware testing. Do I still need an anti-virus protection in Fedora?
No - but not for why you think. While there are antiviruses for *nix systems like Fedora, the threat is very low due in part to better security than windows, and partially due to the lack of profit in writing such things.
If you ran a Windows machine in a VN I would still reccomend using an antivirus, as while the OS is running on 'virtual' hardware it still works in the same way as something running 'on the metal'.
At one level "no" as you can reset the environment should you catch anything - plus you are running a *nix system which is less prone to malware than Windows.
However, if you need something for a longish period then "yes" to avoid having to reset the environment should you catch something.