The test's answer “add-in salt to injury” is a mishearing and combination of two idioms: “add insult to injury,” (to mock, ridicule, or worsen something that is already bad) and “rub salt in a wound” (to worsen physical or emotional pain.)
The misheard phrase, and its variants:
add salt to injury
adding salt to injury
is known as an eggcorn, which has replaced the older term mondegreen.
Although the term mondegreen has been used for misheard phrases not from songs and poems, eggcorn, which originated in a 2003 Language Log post, has been advanced as a broader term for misheard words or phrases that retain their original meanings. So, for example, doggy-dog world is an eggcorn because it’s used in roughly the same way as the original phrase, dog-eat-dog world. Grammarist
In actual fact, the eggcorn has found its way in Google Books, 23 results for “add salt to injury”, and 33 results for “adding salt to injury” While the original and correct idiom, “add insult to injury”, has over 47,000 hits
“Add(-in) salt” sounds very similar to “add insult”, while “injury” is a synonym of “wound”, so it's not surprising people have mixed the two idioms together.