[libcxx-commits] [libcxx] [libc++][math][NFC] Generalize test-suite solution for `signbit` and `fpclassify` tests (PR #217897)

[Lucas Mellone via libcxx-commits]

lknknm wrote:

@frederick-vs-ja I know you are fixing the `signbit` test on your PR https://github.com/llvm/llvm-project/pull/212398, but given the workaround is not yet a consensus, I just wanted to push both at the same time here so it's a smaller context.

https://github.com/llvm/llvm-project/pull/217897