tests: Adding parallelism for sequential ceph-dencoder calls

tests: Adding parallelism for sequential ceph-dencoder calls

The current code was running sequentially two ceph-dencoder calls.
This process is executed pretty fast but adding sequentiality and by the number
of loops to execute, it have a cost.

This patch is just making this two calls being run in parallel.

As a result, the test/encoding/readable.sh test is running in 4m50 instead of 6.
The associate loadavg isn't impacted as it stays at 6 while being run with
nproc=8.

This patch save 1/6th of building time without impact the loadavg.

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