test/mempool: support default mempool

test/mempool: support default mempool

Mempool test currently supports:
 * ring_mp_mc
 * stack

Adding a new default pool options. So, ring* + stack + default
(which can be 'stack' or 'ring')
 * This way, whatever the value of RTE_MBUF_DEFAULT_MEMPOOL_OPS is set,
   it would be verified.
 * even if that means duplicating some test (for example when "stack" is
   set as default and it already part of standard test)

Signed-off-by: Shreyansh Jain <shreyansh.jain@nxp.com>
Reviewed-by: Santosh Shukla <santosh.shukla@caviumnetworks.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>