I have no experience with the TOVA test. But, I do know that no online computer test can accurately tell if you have ADHD. There is a reason that the clinical standards call for questionnaires of the child's parents and teachers.
"In the opinion of one expert panel:18 Computerized tests of attention and vigilance (CPTs)33,40–42 are not generally useful in diagnosis because they suffer from low specificity and sensitivity.43,44." http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2993524/
If you google TOVA, you will find a lot of info - this one seems to nail it pretty well. http://adhdtesting.org/tova.htm
I would be happy to answer any questions you might have relating to symptoms, etc. By the way, if you are an adult trying the test because it is so difficult to get a psyc to believe you - you are not alone.