If the sign tests fail, then your new tests will fail as well, so you will end up with two failed tests. One will report something like

    toMS returns the correct number: Expected 3 to equal -3

and the other will report

    toMS any or all negative input result in negative input: Expected false to be truthy

So the additional test would really only serve to confirm that 3 and -3 differ in sign (but won't actually reflect a unique bug)