app/test: fix memory needs

app/test: fix memory needs

Since commit b91c67e5a693211862aa7dc3b78630b4e856c2af,
maximum number of cores is 128, which has increase
the total memory necessary for a rte_mempool structure,
as the per-lcore local cache has been doubled in size.
Therefore, eal_flags unit test was broken since it needed
to use more hugepages.

Increased memory to 18MB, as that is the actual minimum memory necessary
(depending on the physical memory segments, DPDK may need less memory)

Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>