Being a linguistic pedant firstly....
A "Flight Test" is a test (noun) involving flight (adjective). Therefore it might involve more than one flight; it might also be conducted as part of a flight which has other tasks (including other tests, or other activities).
A "Test Flight" is a flight (noun) for the purpose of test (adjective).
I don't know of any official definition. But it seems to me that's the way we would use those terms.