I think it depends on what the purpose of the test is, what is it assessing – same as what Malcolm has said.
I would say not to give extra time or any other special arrangements for any diagnostic test as these measure specific strengths and difficulties, and highlight areas for intervention..
You would potentially inflate the score and nullify the results if the cohort against which the test is standardised did not receive extra time (or other EAA).
But, If you were giving a history test and a child needed a reader, that would be fair, as you are not assessing the ability to read. But if you are assessing reading, a reading test: if you gave a child reader in order to do a reading test, the score would mean nothing. So in the GCSE English reading test a student normally qualifying for a reader is given more time to try to compensate for the fact that they cannot have a reader, and to try to measure their comprehension skills.