Just take them in the US.
By all means study up over here before you go - it's all multiple choice with a 70% pass mark. You can study (memorize!) to get a pass in one day. If you want to get 95%+ you'll need to study for 2-3 days 😉
The reason they want you to turn up having passed the exam is simply so that you can get in the chopper and start asap, and they don't have to waste time teaching you and signing you off. But unless you have easy access to a testing centre you should just wait: there is one at literally every airport in the US. You'll need an FAA medical anyway, so just plan on turning up a day early and getting the medical and exam done, it'll definitely be cheaper than trying to do it over here.