The most common and most likely answer to this crossword clue is the 15 letter word EXPERIMENTATION
I've seen this crossword clue in Independent
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Buddy explains!

The straight part
We know that "testing" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case testing = experimentation.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 EXPERIMENTATION mean?
EXPERIMENTATION noun- The act of experimenting; practice by experiment.
- A set of actions and observations, performed to verify or falsify a hypothesis or to research a causal relationship between phenomena.