Fix for using sprintf.

Resolves warning:

```
./configure CC="gcc -fsanitize=address" && make
In file included from ./wolfclu/clu_header_main.h:71:
/usr/local/include/wolfssl/test.h:1103:18: error: 'sprintf' is deprecated: This function is provided for compatibility reasons only.  Due to security concerns inherent in the design of sprintf(3), it is highly recommended that you use snprintf(3) instead. [-Werror,-Wdeprecated-declarations]
        strLen = sprintf(serialMsg, " %s", words[3]);
                 ^
```

wolfssl/test.h

@@ -1099,10 +1099,11 @@ static WC_INLINE void ShowX509Ex(WOLFSSL_X509* x509, const char* hdr,
         char serialMsg[80];
 
         /* testsuite has multiple threads writing to stdout, get output
-           message ready to write once */
-        strLen = sprintf(serialMsg, " %s", words[3]);
+         * message ready to write once */
+        strLen = XSNPRINTF(serialMsg, sizeof(serialMsg), " %s", words[3]);
         for (i = 0; i < sz; i++)
-            sprintf(serialMsg + strLen + (i*3), ":%02x ", serial[i]);
+            strLen = XSNPRINTF(serialMsg + strLen, sizeof(serialMsg) - strLen,
+                ":%02x ", serial[i]);
         printf("%s\n", serialMsg);
     }