Add test code for String(StringSumHelper &&) constructor()

Author: giulcioffi

test/src/String/test_String.cpp

@@ -101,3 +101,10 @@ TEST_CASE ("Testing String(const __FlashStringHelper) constructor() with invalid
   arduino::String str1(F(buffer));
   REQUIRE(str1.compareTo("Hello") == 0);
 }
+
+TEST_CASE ("Testing String(StringSumHelper &&) constructor()", "[String-Ctor-13]")
+{
+  arduino::String&& str1 = "Hello";
+  arduino::String str2(str1);
+  REQUIRE(strcmp(str1.c_str(), str2.c_str()) == 0);
+}