Fix cast warnings on test with -1.

Author: dgarske

wolfcrypt/test/test.c

@@ -15427,7 +15427,7 @@ static int simple_mem_test(int sz)
 static wc_test_ret_t const_byte_ptr_test(const byte* in, word32 *outJ)
 {
     wc_test_ret_t ret = 0;
-    volatile word32 j = -1UL; /* must be volatile to properly detect error */
+    volatile word32 j = (word32)-1; /* must be volatile to properly detect error */
 
     ret = (wc_test_ret_t)*in; /* accessed *in value. */
     (void)ret;