[libc++][math][NFC] Generalize test-suite improvement for `signbit` and `fpclassify` tests (#217897)
This patch generalizes the test-suite improvements for
`signbit.pass.cpp` and `fpclassify.pass.cpp` as per the review comments
present in https://github.com/llvm/llvm-project/pull/210075 for
consistency purposes.
Follows-up:
https://github.com/llvm/llvm-project/pull/210075#discussion_r3629842324