Dyslexia testing centers are somewhere where you can get dyslexia assessment. In other words, you go there to find out whether you have dyslexia.
Many dyslexia centers provide services of dyslexia assessment. However, you should know that not all such centers' diagnosis can be official. Only when the assessor is a licensed professional can the diagnosis be official.
So, if you want a diagnosis, the best choice is actually finding a licensed psychologist.
Keyword: dyslexia testing centers