librbd/test_librbd.cc: free memory in test_list_children()

CID 719581 (#7 of 7): Resource leak (RESOURCE_LEAK)
CID 719581 (#6 of 7): Resource leak (RESOURCE_LEAK)
  leaked_storage: Variable "pools" going out of scope leaks the
  storage it points to.
CID 719582 (#6-7 of 7): Resource leak (RESOURCE_LEAK)
  leaked_storage: Variable "children" going out of scope leaks
  the storage it points to.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>