b12175

tests : override hardware_concurrency in CI (#31797)

The self-hosted CPU runners are containers with 8 CPUs, but
std::thread::hardware_concurrency reports the cores of the host. The
table mode of test-llama-context sizes its pool from that count, so it
runs 4 models x 8 threads on an 8-CPU job and hangs.

Add TEST_ARG_HARDWARE_CONCURRENCY and TEST_ARG_PARALLEL_WORKERS as the
defaults for the auto-selected counts of test-llama-context and of
test-backend-ops, and set them to 8 and 4 for cpu-x64-high-perf.

Assisted-by: dsh:llama.cpp/ggml-org/Qwen3.8-Flash-Next-GGUF (RTX 5090 32GB, DDR5 128GB)

Website:

• https://llama.app

Attestations:

• https://github.com/ggml-org/llama.cpp/attestations/64562762

macOS/iOS:

• macOS Apple Silicon (arm64)
• macOS Apple Silicon (arm64, KleidiAI enabled) DISABLED
• macOS Intel (x64)
• iOS XCFramework

Linux:

• Ubuntu x64 (CPU)
• Ubuntu arm64 (CPU)
• Ubuntu s390x (CPU)
• Ubuntu 26.04 s390x (CPU)
• Ubuntu 26.04 s390x (zDNN)
• Ubuntu x64 (Vulkan)
• Ubuntu arm64 (Vulkan)
• Ubuntu x64 (CUDA 12) - CUDA 12.8 libraries
• Ubuntu x64 (CUDA 13) - CUDA 13.5 libraries
• Ubuntu arm64 (CUDA 13) - CUDA 13.5 libraries
• Ubuntu x64 (ROCm 10.0)
• Ubuntu x64 (OpenVINO)
• Ubuntu x64 (SYCL FP32)
• Ubuntu x64 (SYCL FP16)
• Linux arm64 (Snapdragon: CPU, Adreno GPU, Hexagon NPU) - setup guide

Android:

• Android arm64 (CPU)
• Android arm64 (Snapdragon: CPU, Adreno GPU, Hexagon NPU) - setup guide

Windows:

• Windows x64 (CPU)
• Windows arm64 (CPU)
• Windows arm64 (OpenCL Adreno)
• Windows x64 (CUDA 12) - CUDA 12.4 DLLs
• Windows x64 (CUDA 13) - CUDA 13.5 DLLs
• Windows arm64 (CUDA 13) - CUDA 13.5 DLLs
• Windows x64 (Vulkan)
• Windows arm64 (Vulkan)
• Windows x64 (OpenVINO)
• Windows x64 (SYCL)
• Windows x64 (ROCm 10.0)

openEuler:

• DISABLED
• openEuler x86 (310p)
• openEuler x86 (910b, ACL Graph)
• openEuler aarch64 (310p)
• openEuler aarch64 (910b, ACL Graph)

UI:

• UI