AFFFFFTTTTTTTERRRRRRRRRR!
A quality exam is hard to come by, and there is no need to waste a good test before you are in a position to learn from taking it and reviewing it.
A bad exam taken to start can send you off in the wrong direction, worried about the wrong things.
Either way, good or bad exam, taking it right out the gate is bad.
There is no such thing as a "diagnostic exam", because it would require at least 1200 questions to cover everything the MCAT covers in terms of subjects. Add in that many questions we miss are the result of test taking issues (anxiety, misreading, premature answer selection, etc...), and no diagnostic exam can account for that.
The best place to start, if you want anything remotely helpful like a diagnostic exam, is with AAMC's Official MCAT Guideline. It tells you exactly what is tested and gives you a great model of how exam passages and questions are written.
Once you have reviewed material and done plenty of practice questions, then try a FL.
Good luck!