I like this idea! The way we can get started very quickly is to post individual threads where everybody will post related test questions with multiple choice options. When we have 10-15 questions and answers ready, we can convert them into an interactive test using our own or other online tools.

For example, someone starts a thread called "Test on Russian Cases (Beginning Level)". Then we have everybody contribute test questions like this:

Я ищу ....

1. работу
2. работе
3. работой
4. работ