kselftest/arm64: Ensure stable names for GCS stress test results

The GCS stress test program currently uses the PID of the threads it
creates in the test names it reports, resulting in unstable test names
between runs. Fix this by using a thread number instead.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20241011-arm64-gcs-stress-stable-name-v1-1-4950f226218e@kernel.org


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>