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>
Well it's very hard to synchronize concurrent execution to start at the exact same time even in real-multi-thread/processes environment. That's why I have multiple workers, which queue up multiple jobs, which means there are jobs started earlier, put into background and different one is started.... So don't worry, it puts significant stress too.

Anyway you are right, that it might not uncover all bugs (eg. if the locking is almost correct, but there is 0.000000001s where it can be screwed, that this might not uncover it as often as precisely queued jobs. Anyway even precisely queued jobs executed at the same time might not reach the lock part at the same time, so let's rely in environmental delays...)