Along the way, modify the piglit run script and refactor the way
piglit jobs are generated.

v2:
  - Squashed the commit to remove tracie jobs (Eric).

v3:
  - Extend information in the comments about the need to use a
    running X server for replaying with Vulkan (Tomeu).
  - Do actually fail if the upload doesn't work (Tomeu).

v4:
  - Rename *-piglit-traces jobs with *-traces.

Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com> [v3]
Part-of: <!6388>