If something is testing, it is hard, and it is hard in a way that finds out how strong you are. A testing week at work, a testing time after someone dies, a testing question in an interview. The word carries the idea of a test: the difficulty is not just unpleasant, it shows what you are made of. Because of that, we usually say it about things that go on for a while and that a person must get through — a testing period, testing conditions, a testing few months. We do not usually say it about one small annoying moment; a broken zip is annoying, not testing. It is a little formal and rather British. You will meet it in newspapers, in reports, and in the polite speech of someone who does not want to say 'terrible' — 'It has been a testing year for the company' means the year was bad, said gently. Two more things worth knowing. First, it often carries a quiet compliment: calling a time testing suggests the person came through it, or is expected to. Second, sports writers love it — 'a testing match', 'testing conditions' — where it means the other side, or the wind and rain, made the players work hard. Note that this is an adjective, and it comes before the noun far more often than after it. 'A testing question' sounds normal; 'the question was testing' is possible but less common, and many speakers would say 'the question was difficult' instead.