Your test is intended to convey to the reader that your function does addition and provide an example of how to access that function. The functional part of the test is to verify that your claim is true, rather than the stories of yore when the documentation often didn't match what the code actually did.

As a nice side effect, when you refactor your function in the future and you mistakenly turn it into multiplication, the test will give you a sanity check that you meant for it to do addition. Perhaps not a big deal in your contrived example, but is huge in the real world where functions are not quite so simple.

Testing is not for verifying a function is mathematically correct.