Depends what you mean by "bad luck". What sort of "driving test failing scenario" would involve failing because of bad luck?
I'd argue that it's probably just as easy to pass the test as it is to fail. What's the national first-time pass rate? About 45-46%?
The issue of being nervous for a driving test is a difficult one though, that's for sure. And I'm not sure there's an answer to that. Apart from the fact that if you're nervous, you're probably not confident that you'll pass, which means you should go and practice until you ARE confident that you'll pass.
*shrugs*
So whilst on your test you were absolutely 100% confident in your ability and knew without a doubt that you would pass your test? There is nothing wrong with being nervous, it's a basic human reaction. Probably better to be nervous than over confident.