test/malloc: run realloc tests on external heap

test/malloc: run realloc tests on external heap

Due to the fact that the rte_realloc() test depends on the layout of
underlying memory, it can sometimes fail due to fragmentation of the
memory. To address this, make it so that the realloc autotests are run
using a newly created external memory heap instead of main memory.

Bugzilla ID: 424

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Tested-by: David Christensen <drc@linux.vnet.ibm.com>