ci/deqp: Allow specifying the caselist fraction separate from CI_NODE_INDEX.

ci/deqp: Allow specifying the caselist fraction separate from CI_NODE_INDEX.

To increase our VK coverage on a630, we want to have two jobs in parallel,
but we still can't hit full coverage so we need the fractional setting to
be separate from gitlab CI's flags for setting up parallel jobs.

Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6971>