It's possible, but unlikely. 'At ease' in that context is a military stance, so it would make sense if you were referring to a soldier doing a test while standing in the 'at ease' position. I guess that's not what you meant. I assume you meant "He did the test with ease".

You could say that although it would be a little uncommon. He did the test at ease = He did the test calmly/with no worries He did the test easily = He did the test with no effort/without any problems

Yes, that would work. "At ease" usually collocates with "feel".

Thank you,Su.Ki. What about "He felt at ease when taking the test"?

I agree with Gary that 'at ease' would not be a natural expression. If you wrote this in an English test, you might lose a mark for using a phrase which was unclear or inappropriate. "At ease" does mean 'relaxed', but it would be a very odd phrase to use in this context. If you mean that the test was easy for him, you have to use the adverb 'easily', NOT 'at ease'.