What’s the proper name for code that tests another code?

That seems like a stupid question, but I can’t find a proper name for “code that tests another code”. The most of literature names that kind of code just “tests” but it’s way to general in my understanding (obviously I am not an English native speaker so I might be wrong) – code might be tested by people, some external machine that is not “code” at all. The same applies to “code tests”.

I was thinking about “testing code” but this seems to be a bit confusing as it looks more like a verb than noun.