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)
