Fix stats output, exit code upon errors and parallelism in runConformance.py

This MR fixes three usability problems with scripts/ivas_conformance/runConformance.py:

• the output of stats() is truncated when the command returns because the shell prompt is written over it
• the script silently fails when issued test commands fails
• the multiprocessing was implemented without pooling which can cause OSError: [Errno 24] Too many open files

CC @kiene