Tests: Compare infinities and zero signs exactly in ULP checks

ulp_distance folded -0 onto +0 and put an infinity one step from the largest
finite value, so every ULP budget accepted +0 for sqrt(-0), and any budget of
one or more accepted overflow. An infinity is now as far as possible from any
other value, and so is a zero from the zero of the other sign; the pmadd
check inherits the exact zero sign.

Validation: C++14, Release, seed 1. GPU parts 1, 3 and 7 pass with GCC 13.3 /
NVCC 12.8 and with Clang 20 / CUDA 12.8 (-ffp-contract=fast) on an NVIDIA
GeForce RTX 4070 Laptop GPU (sm_89) under WSL2. GCC 13.3 passes all 15
packetmath parts, the four special_packetmath parts and packetmath_fastmath.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>