ci: riscv: partition tests and add timeouts for QEMU stability

[allnes]

Motivation

The RISC-V CI runs the test suite under QEMU user-mode emulation, which is ~10-50x slower than native. The vlen256 job occasionally hangs: in one recent run it ran ~58 min and the Run oneDNN tests step produced no output at all before the job died, giving zero diagnostic signal. Normal runs finish in ~17 min (181 tests), so this is a transient QEMU/runner stall rather than a real test failure — but the silent multi-hour hang and the lack of a per-test cap make it hard to diagnose and slow to fail.

This is a RISC-V CI robustness change only — no library code is touched.

Changes

.github/workflows/ci-riscv.yml

• fail-fast: false on the test matrix so a flaky vlen128/vlen256 run (or one partition) no longer cancels the siblings.
• timeout-minutes: 40 on the test job — a hung QEMU run is now killed promptly instead of stalling toward the 6 h default.
• Split each vlen config across 4 parallel partitions via a part/parts matrix dimension (2 vlen × 4 = 8 jobs). Wall-clock per partition drops ~linearly.

.github/automation/riscv/test.sh (SMOKE branch)

• Apply stride partitioning ctest -I ${start},,${stride} driven by ONEDNN_TEST_PART/ONEDNN_TEST_STRIDE. This mirrors the partitioning the CI testset branch already uses; stride needs no cost table and covers every test exactly once (verified: N=181 → 46/45/45/45, no gaps, no overlaps).
• Add --timeout ${ONEDNN_TEST_TIMEOUT:-600} so a hung test (e.g. an RVV codepath that loops forever) fails fast with its name instead of stalling the whole job. SMOKE tests run in seconds, so 600 s is a generous cap. The cap is scoped to the SMOKE branch only and does not affect the weekly CI testset (which has multi-hour tests).

Notes

• Defaults preserve old behavior when the env vars are unset (start=1, stride=1 → all tests, single partition).

[allnes]

@aobolensk please review

[aobolensk]

I don't think this is the right place to add such change. Let's submit it to https://github.com/uxlfoundation/oneDNN first and then cherry-pick it