[libc][NFC] add an atof test for a fuzz failure

The differential fuzzer found that glibc and our libc disagree on the
result for "0x30000002222225p-1077", with ours being rounded up and
theirs rounded down. Ours is more correct for the nearest rounding mode,
so only a test is added.

Reviewed By: lntue, sivachandra

Differential Revision: https://reviews.llvm.org/D145821