Reset the whole stack in testutils

Reset the whole stack in testutils

The memset() was resetting only 30 bytes in the array (size of the
array), but it is array of pointers.  Since it is a static array,
let's just reset it by its size.

Found by gcc-7.1:

  testutils.c: In function 'virTestRun':
  testutils.c:243:13: error: 'memset' used with length equal to number
  of elements without multiplication by element size [-Werror=memset-elt-size]
    memset(testAllocStack, 0, ARRAY_CARDINALITY(testAllocStack));
    ^~~~~~

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>