A tester who understands, knows, and has ample experience for the whole gifted range is very important, imho.

They will know how to make the test itself more enjoyable and bearable for the child. With great ability to motivate if child feels onset of perfectionism and doesn't want to guess, the tester will know how to deal with that. Or, if the questions start off too easy, the tester will know how to motivate the child to try.

A warm fun tester personality is a definitely nice to have too.