> how is there not an automated set of tests that use one out of every other IEEE754-associated type as input to then check whether it generates output which falls outside the assumed set of types?

It's slightly worse than that. Math.expm1(-0) = -0 is explicitly called out as part of the standard[1]. You don't need to throw every IEEE754 at your functions to make sure they're all correct, but validating the explicitly defined corner cases is table stakes.

1. https://www.ecma-international.org/ecma-262/6.0/#sec-math.ex...