app/test: migrate custom mempool handler to stack handler

app/test: migrate custom mempool handler to stack handler

After introducing the stack handler in the previous commit,
we now have very similar code to the custom handler in test_mempool.c,
which creates a custom mempool based on simple mallocs.
The stack handler is a cleaner example of adding a new mempool handler,
so this commit replaces the custom handler test with a stack
handler test, and removes the custom handler code.

Signed-off-by: David Hunt <david.hunt@intel.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>