tests.doublefree: Add 'double free' test.

Double free performs a build of doublefree.c and execution,
that triggers a double free() function calling, for the same malloc()
region.

The motivation behind this test is to catch libc errors and log inside
Avocado execution (see bug #79).

Signed-off-by: Ruda Moura <rmoura@redhat.com>
Merge pull request #85 from ruda/doublefree_test

tests.doublefree: Add 'double free' test.