tests: getting paraller_jobs more robust

tests: getting paraller_jobs more robust

This patch fixes several issues:
 1. lscpu may not exist on all systems.
 2. Option -b is not always available but we can skip it because it's
    the documented default for -p.
 3. Some old lscpu versions print --help output to stdout in case of
    error. So in case 2. it would count all the help lines as CPUs,
    which could be really bad on slow single core machines.

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>