According to the Guide for teachers (page 11) at ielts.org:
IELTS recognises both British and American English spelling, grammar and choice of words. It also incorporates a mix of native speaker accents from Australia, Canada, New Zealand, the UK and US in the Listening component.
Despite some regional differences, Australians, Americans, British, Canadians and New Zealanders can more or less easily understand each other’s English (assuming regional slang expressions and technical jargon are not used!). Examiners should accept any of those regional varieties.
Using past simple instead of present perfect
I notice that Americans commonly use past simple (“I saw it already”) where present perfect (“I’ve seen it already”) is traditionally used.
However, an American English teacher I work with once noticed this grammar variation used in a textbook, and insisted that he would not use it himself.
So, you should probably avoid using the American variation in the IELTS test, unless your examiner has already done it (or already did it?) before you do.