"Palabra usada en una prueba/en un examen" is another possibility. I hope this helps.

Depending on context, it could be:
'palabra de prueba', 'palabra de la prueba' (if test is a noun);
or 'probar palabra','pruebe la palabra'(if test is a verb).

Magdalena Smoot

The answer may be as simple as that if you are talking about the context of testing as in informational technology. In English we would use the phrase test word to indicate only that it is a word - as opposed to numbers or a simple string of data - being used. Quite often the IT folks that I work with will use just the word test to indicate that something must be sent through the program to test it, whether it is a word or a data string of gibberish. Hope it helps.