apparmor: Fix uninitialized symbol 'array_size' in policy_unpack_test.c

Make sure array_size is initialized in the kunit test to get rid of
compiler warnings. This will also make sure the following tests fail
consistently if the first test fails.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>