[libc++][test] `fp_compare.h` includes non-portable `<__config>`

[StephanTLavavej]

Introduced by #100820:

llvm-project/libcxx/test/support/fp_compare.h

Line 15 in 52a7116

#include <__config>

Found while running libc++'s tests with MSVC's STL, which naturally lacks this libc++ internal header.