app/test: rework mempool test
app/test: rework mempool test

Rework the mempool test to better indicate where it failed,
and, now that this feature is available, add the freeing of the
mempool after the test is done.

Signed-off-by: Olivier Matz <dfcc1510895413197abb4336e39bb0a3906cba71@6wind.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk