I'm well known for being a miserable doom-and-gloom merchant. So don't be surprised:
If an engineer wants to test something, it's because there's a possibility the something could fail. So, a situation is chosen to reduce risk but still providing a valid test scenario. Common sense.
So, the Thames Barrier is to be tested when there's 1000+ vessels nearby. And if they have to adjust the Barrier because of a problem ...