The most common and most likely answer to this crossword clue is the 4 letter word VIVA
I've seen this crossword clue in The Daily Telegraph
V
I
V
A
Buddy explains!

The straight part
We know that "test" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case test = viva.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does VIVA mean?
VIVA noun- A shout of applause.
- To cheer, applaud
- Long live ... ! (used to express acclaim or support).
- An oral examination, typically for an academic qualification.
- To subject to an examination of this kind.