[libc-commits] [libc] [libc][math] implement `signbit` and math macro unit tests (PR #97791)

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akielaries wrote:

Makes sense. Original issue #96322 had mentioned 

> It seems that we don't have tests for isfinite, isinf, or isnan macros from our math.h, which is problematic. We should start a new "include" test under libc/test/include (similar to stdbit_test.cpp).

So I figured closing out the issue in one PR was reasonable. I had also planned to remove the TODO that was in `math-macros.h` for moving `isnan`, `isinf`, `isfinite` to a new file (didn't see another issue(s) directly mentioning these individually) but looks like #96008 was addressing that

https://github.com/llvm/llvm-project/pull/97791