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: NMartin Kletzander <mkletzan@redhat.com>