tests: add test for output to std::cout

Author: igrr

tests/device/test_iostream/test_iostream.ino

@@ -0,0 +1,34 @@
+#include <ESP8266WiFi.h>
+#include <BSTest.h>
+#include <sstream>
+#include <iostream>
+
+BS_ENV_DECLARE();
+
+void setup()
+{
+    Serial.begin(115200);
+    BS_RUN(Serial);
+}
+
+TEST_CASE("can print to std::cout", "[iostream]")
+{
+    std::stringstream test_stream("");
+    test_stream << "hello stream";
+
+    // empty the RX buffer, just in case
+    Serial.readString();
+
+    USC0(0) |= (1 << UCLBE);  // enable loopback
+    std::cout << test_stream.str().c_str() << std::endl;
+    delay(100);
+    USC0(0) &= ~(1 << UCLBE);  // disable loopback
+    
+    String result = Serial.readStringUntil('\n');
+
+    Serial.printf("result: '%s'\n", result.c_str());
+
+    CHECK(result == test_stream.str().c_str());
+}
+
+void loop() { }