Fix: tests: don't assume sequential cpuids

On Linux CPU ids aren't sequential if a CPU is offlined or unplugged.
Get the list of currently available CPU ids from sysfs and pick a random
one, if sysfs is not available use the previous behavior.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ibdb63c7d036389104ac2f629827a6dce59e06983