subtests.docker_cli.save_load: Add stressed_load subsubtest

This pull request refactors the existing test a bit and adds another variant asked by https://github.com/autotest/autotest-docker/issues/248 . I changed the commands to use `sh` instead of `bash` as they don't use anything fancy and `bash` is not present in `busybox` image, which was the only one I could use for testing on my Fedora with `tmpfs` `/tmp` with limited free space...

This subsubtest spawns multiple workers and simultaneously loads
multiple existing and new images.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>