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: NMartin Kletzander <mkletzan@redhat.com>

tests/testutils.c

@@ -240,7 +240,7 @@ virTestRun(const char *title,
         for (i = start; i < end; i++) {
             bool missingFail = false;
 # ifdef TEST_OOM_TRACE
-            memset(testAllocStack, 0, ARRAY_CARDINALITY(testAllocStack));
+            memset(testAllocStack, 0, sizeof(testAllocStack));
             ntestAllocStack = 0;
 # endif
             virAllocTestOOM(i + 1, 1);