test/distributor: collect return mbufs

test/distributor: collect return mbufs

During quit_workers function distributor's main core processes
some packets to wake up pending worker cores so they can quit.
As quit_workers acts also as a cleanup procedure for next test
case it should also collect these packets returned by workers'
handlers, so the cyclic buffer with returned packets
in distributor remains empty.

Fixes: c3eabff124e6 ("distributor: add unit tests")
Fixes: c0de0eb82e40 ("distributor: switch over to new API")
Cc: stable@dpdk.org
Signed-off-by: Lukasz Wojciechowski <l.wojciechow@partner.samsung.com>
Acked-by: David Hunt <david.hunt@intel.com>