tests: set MALLOC_PERTURB_ to expose memory bugs

glibc wipes malloc(3) memory when the MALLOC_PERTURB_ environment
variable is set.  The value of the environment variable determines the
bit pattern used to wipe memory.  For more information, see
http://udrepper.livejournal.com/11429.html.

Set MALLOC_PERTURB_ for gtester and qemu-iotests.  Note we pick a random
value from 1 to 255 to expose more bugs.  If you need to reproduce a
crash use 'show environment' in gdb to extract the MALLOC_PERTURB_
value from a core dump.

Both make check and qemu-iotests pass with MALLOC_PERTURB_ enabled.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1369661331-28041-1-git-send-email-stefanha@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
