test/distributor: ensure all packets are delivered

test/distributor: ensure all packets are delivered

In all distributor tests there is a chance that tests
will send packets to distributor with rte_distributor_process()
before workers are started and requested for packets.

This patch ensures that all packets are delivered to workers
by calling rte_distributor_process() in loop until number
of successfully processed packets reaches required by test.
Change is applied to every first call in test case.

Cc: stable@dpdk.org
Signed-off-by: Lukasz Wojciechowski <l.wojciechow@partner.samsung.com>
Acked-by: David Hunt <david.hunt@intel.com>