Does a person's not feeling tested in their life truly mean they are not a believer?

A Muslim adheres to Allah's commands and abstains from His prohibitions. He should not hasten tribulation or wish for it. Indeed, the Prophet, peace and blessings be upon him, forbade wishing to meet the enemy and commanded seeking well-being, as he said: «O people, do not wish to meet the enemy, and ask Allah for well-being.» He also urged seeking refuge from severe tribulation, evil decree, and the gloating of enemies. The Prophet, peace and blessings be upon him, rebuked a man who wished for the hastening of the Hereafter's punishment in this world and guided him to ask Allah for well-being in this world and the Hereafter.

As for the hadiths regarding the virtue of patience in tribulation, they encourage patience after the tribulation has occurred, not the seeking of it.

Well-being is one of Allah's greatest blessings, and a Muslim must be grateful to Allah for it. Absolute well-being includes well-being from disbelief, transgression, and disobedience. The Prophet, peace and blessings be upon him, used to supplicate with it morning and evening, and in his qunoot. We have been commanded to ask Allah for pardon and well-being, for no one has been given anything better than well-being after certainty.

Gratitude for the blessing of well-being is a great door of servitude. Allah has commanded it and promised the grateful increase. Gratitude is half of faith and is one of the highest stations. Allah has described His prophets with it and called Himself the Most Grateful. He has made the grateful ones those who benefit from His signs. It indicates the virtue of gratitude that the Prophet, peace and blessings be upon him, stood until his feet swelled, and he said: «Should I not be a grateful servant?»