What's known as 'standard English' is a dialect. It's the dialect that was spoken by the British ruling classes a century or so ago, and is still the preferred option for many.
IELTS would be valid regardless of whether there really is a standard English: IELTS students learn effective communication in the dialect that is most widely recognised and understood.
I don't really see how one can talk of one dialect versus English itself. There is no 'pure' form of a language: usage dictates grammar, not the other way about. To put it another way, grammar rules are descriptive, not prescriptive.
Of course, we often loosely talk of 'pure', 'correct', 'standard', etc., but really these terms refer only to what is widely accepted, and this can and does change.
To suggest that there is, or was, a core of English that dialects can approach or move away from, is not really accurate. There is/was one dialect that is/was accepted in influential circles.