tests/mempool: Allocate pool in child for robust tests

tests/mempool: Allocate pool in child for robust tests

Depending on the populate policy, it is not possible to use a pool
across a fork.  Since robust tests rely on SIGABRT to be triggered in a
child process, the pool must be allocated in the child instead of the
parent for the RSEQ_MEMPOOL_POPULATE_PRIVATE_NONE policy.

Change-Id: I78bc19d12b4f8bc0c7e2a2f05fab870ed7524e13
Signed-off-by: Olivier Dion <odion@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>