Improve math tests to allow a specific ulp bound.

At the moment our libm is only good enough for a 1 ulp bound on these tests,
but that's better than the 4 ulp bound you get from gtest by default.

I'm not really happy with the multiple structures and corresponding functions,
but at least they mean there's no duplication in the tests themselves, and it
should be easy enough for us to make further improvements in future.

Change-Id: I004e12970332e1d9531721361d6c34f908cfcecc