It is perfectly consistent for something to be well-defined and also untestable.
For example, you could ask me any question about the contents of my mind. Let's say you ask me if my favourite colour is green, and I say it is. You don't know whether my favourite colour really is green or if I'm lying. The fact that you have no way to ascertain my true favourite colour doesn't mean favourite colours are ill-defined! You just can't know what anyone else's favourite colour is.
(Not that I'm claiming my "subjective experience" definition is well-defined -- I accept that it's pretty vague as definitions go).
For example, you could ask me any question about the contents of my mind. Let's say you ask me if my favourite colour is green, and I say it is. You don't know whether my favourite colour really is green or if I'm lying. The fact that you have no way to ascertain my true favourite colour doesn't mean favourite colours are ill-defined! You just can't know what anyone else's favourite colour is.
(Not that I'm claiming my "subjective experience" definition is well-defined -- I accept that it's pretty vague as definitions go).