Hi,
Thanks for visiting this thread!
Your case is now becoming more common than you think. I dare say you have nothing to worry about. As a matter of fact I think it is a strong positive sign that your visa will be approved. The embassy does not contact people who are likely to be refused about this.
Secondly, I personally think it is an unreasonable request. They do not specify if the exam to be take should be the general or academic test. They also do not put into consideration the logistics of having to register for the test, preparing for the test, waiting for the results etc 30 days is too small for that. Furthermore, it is unreasonable for them to stop processing the applicant's case for 30 days pending when those documents can be provided.
Nevertheless, it is up to you. If you want to register and take the test its fine, if not, just disregard the mail, at the end of 30 days, they will assume you are not interested in that route and then continue to process your application.
In my personal opinion it is nothing to be worried about.
I wish you the very best.
Omofar