tcp_cmd

add pbuf_copy_partial moc so I don't need if TESTING preprocessor directive

tests/mocs.c

@@ -148,4 +148,9 @@ size_t tcp_sndbuf(void* pcb) {
 
 void pbuf_free(struct pbuf* p) {
     UNUSED(p);
+}
+
+uint16_t pbuf_copy_partial(const struct pbuf* buf, void* dataptr, uint16_t len, uint16_t offset) {
+    memcpy(dataptr, ((uint8_t*)(buf->payload)) + offset, len);
+    return len;
 }