Well, according to the Bible, the test of a prophet is clearly set out in Deuteronomy 18:22
"When a prophet speaketh in the name of the LORD, if the thing follow not, nor come to pass, that is the thing which the LORD hath not spoken, but the prophet hath spoken it presumptuously: thou shalt not be afraid of him."
In other words if what's been prophesied fails to take place, it leaves us with a false prophet.
So why is it that Jonah is referred to as a prophet?
Jonah prophesied that Nineveh would be overturned in 40 days.
The Ninevites believed this and commenced a fast as well as changing their clothes. Even the king got in on the act by ordering his people to wear sackcloth and not to eat or drink.
"So the people of Nineveh believed God, and proclaimed a fast, and put on sackcloth, from the greatest of them even to the least of them. For word came unto the king of Nineveh, and he arose from his throne, and he laid his robe from him, and covered him with sackcloth, and sat in ashes. And he caused it to be proclaimed and published through Nineveh by the decree of the king and his nobles, saying, Let neither man nor beast, herd nor flock, taste any thing: let them not feed, nor drink water: But let man and beast be covered with sackcloth, and cry mightily unto God: yea, let them turn every one from his evil way, and from the violence that is in their hands. Who can tell if God will turn and repent, and turn away from his fierce anger, that we perish not?"
Jonah 3:6-9
The people of Nineveh repented and God refrained from visiting destruction on Nineveh.
"And God saw their works that they turned from their evil way; and God repented of the evil, that he had said that he would do unto them; and he did it not."
Jonah 3:10
So the destruction of Nineveh never came to pass.
Jonah got it wrong - he was no prophet.