ci/piglit: support parallel in piglit jobs

This series allows using parallel: in piglit jobs, similar to deqp-runner based jobs.

This only works when using a single profile, and unfortunately it doesn't work with replay profile. But works fine with the other profiles. If you try to use parallel or fraction with replay or multiple profiles, an error will be raised in the runner.

The other change required was splitting PIGLIT_OPTIONS in two: on one side, PIGLIT_TESTS contains the tests to include or exclude (with the -x or -n selector); the rest of the piglit options will be kept in PIGLIT_OPTIONS.