Do scientist's use endangered animals for animal testing?

There isn't any.

To do animal testing, you need quite a lot of animals to get reliable results.

What happens to ONE animal, once, might just be random.

And if something should be discovered later on, someone might need to go back and repeat the tests.

Endangered animals are animals that are rare.

Either hard to find, or not many left, or both.

Ignoring everything else, it'd simply be hugely impractical to use endangered animals for animal testing.