I think this can be expanded beyond just following up after a test. I would say that in general, it is worthwhile to either send a followup email or make a followup call when it has been more than a week since hearing from a potential employer during the hiring process.
The worst thing that happens is they tell you they aren't interested in hiring you, and if that is the case then it is better to know than not know.
The list of good things that could happen from a followup is long. It shows them that you care about getting that job, it may push them to making a decision more quickly, it may remind them to take a look at your test results, and many more things I am probably not thinking of.
Now that said make sure you word the followup in a way that doesn't make it seem like you are trying to rush them or are getting impatient. Something like "Hello, this is [insert name here], I haven't heard from you since my last email so I just wanted to contact you and make sure you had a chance to look over my online test results" is a lot more likely to evoke a positive response than "Hey, why haven't you gotten back to me yet?"