Make tests work on public repo as well

PUBLISHED_FROM=78acb18d597b6f47f31da13087fa6685f21ce445

src/common/platforms/esp32/stubs/uart.c

@@ -0,0 +1,15 @@
+/*
+ * Copyright (c) 2014-2016 Cesanta Software Limited
+ * All rights reserved
+ */
+
+#include "uart.h"
+
+#include "rom_functions.h"
+
+void set_baud_rate(uint32_t uart_no, uint32_t baud_rate) {
+  uint32_t master_freq = ets_get_detected_xtal_freq() << 4;
+  master_freq += (baud_rate / 2);
+  uint32_t div = master_freq / baud_rate;
+  uart_div_modify(uart_no, div);
+}