ci/piglit: allow parallel piglit jobs

ci/piglit: allow parallel piglit jobs

This allows to split a piglit job in several parallel jobs, to speed up
the execution.

Due piglit restrictions, this only works for single profiles. Otherwise
an error will be shown in the runner.

Also, a new gitlab job variable `PIGLIT_TESTS` is introduced that
contains the excluded/included tests with `-x` or `-n`. The rest of the
piglit options go to `PIGLIT_OPTIONS` (like `--timeout n`).

v2 (Andres):
 - Replay profile is supported in parallel jobs.
 - Bail out inmediately if parallel jobs is tried with multiple
profiles.
 - Use testlist only when doing parallel jobs.
 - Do not drop pass tests when filtering executed tests.
 - Get rid of PIGLIT_FRACTION.

v4:
 - uncommit unrelated change (Andres).

Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>
Reviewed-by: Andres Gomez <agomez@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9022>