To the OP, those who aspire...
The term "test pilot" has a core understanding, though has a broader interpretation too. There is no one kind of test pilot, nor one kind of aircraft to be tested. Thus there can be no one kind of test pilot - it takes many.
I don't know of a definition for test pilot. I am certainly subject to highly qualified opinion here, but I will suggest that "test pilot" could mean that you act as a flight crew member, as directed by a competent entity, to evaluate, confirm, or demonstrate compliance toward certifiction or acceptance of a type or a modification.
Test pilots will have in common a serious and inquisitive interest in how and why aircraft fly, and how they can contribute to defining that, and making it better, or at least documenting its compliance.
Test flying is not about looking for excitement - it's more about enduring some excitement, so the pilots who fly that type will not be exposed to surprises.
A test pilot has missed the mark, if an operational crew comes back, and draws their attention to a meaninful flight characteristic, which they had not detected within the scope of their testing. I have been in this situation twice. Niether were "serious" - but they did remind me to be more vigilent.
There is a pride in flight testing an aircraft, and adding value. Resist the notion of being seen by an adoring audience climbing into the cockpit with your white scarf, that can lead to sudden ego adjustment, when something does not go as planned.
I enjoy test flying modified aircraft (I don't test new types, just certified aircraft with modifications), but in truth, taking a freind, or family member on an adventure in my plane is just as much fun!