C++: Add a test of append with CharT.

Author: geoffw0

cpp/ql/test/library-tests/dataflow/taint-tests/stl.cpp

@@ -366,4 +366,12 @@ void test_string_append() {
 		s9.append(" ");
 		sink(s9); // tainted
 	}
+
+	{
+		std::string s10("abc");
+		char c = ns_char::source();
+
+		s10.append(1, c);
+		sink(s10); // tainted [NOT DETECTED]
+	}
 }