Tests: Introduce conf_proc_count()

Tests are using the nproc utility which return the number of
_available_ CPUs. The distinction between online, available and
configured CPUs is subtle, but important.

The number of "online" CPUs can change at runtime as CPUs are
hot-plugged. This could happen during a test and result in
unexpected results.

The number of "configured" CPUs includes any CPU which may be
offline at the time.

The number of "available" CPUs, which is what is returned by the
nproc utility, may differ from both "online" and "configured"
counts. This is the case in containers which are assigned to a
subset of configured CPUs.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>