tests: Adding parallelism to check-generated.sh

tests: Adding parallelism to check-generated.sh

This script had the following performance issue :
- 4 ceph-dencoders spawn sequentialy
- running twice the same dencoder command

This patch is adding parallelism around the 4 sequential calls but also
prevent from testing the deterministic feature twice.

On a recent laptop, this patch drops the running time from 7mn to 3m46
while keeping the loadavg < 2.

Signed-off-by: Erwan Velu <erwan@redhat.com>