test: fix autotest crash

test: fix autotest crash

On some systems when dpdk test is executed with make test command
autotest_runner crashes in first_cpu_on_node. This happens when list
of available cpus contains something that is not a cpu as first element.
Fixed by removing all non-cpu values from list of available cpus.

Bugzilla ID: 253
Fixes: 22dcd9a4d90f ("test: parallelize unit tests")
Cc: stable@dpdk.org
Signed-off-by: Herakliusz Lipiec <herakliusz.lipiec@intel.com>
Reviewed-by: Anatoly Burakov <anatoly.burakov@intel.com>