C#: Add UTF-8 encoded string test case.

Author: michaelnebel

csharp/ql/test/library-tests/csharp11/Strings.cs

@@ -35,4 +35,14 @@ spans multiple lines.
         Show matching set of curly braces: {{{message2}}}
         """;
     }
+
+
+    public void M3()
+    {
+        // UTF-8 encoded.
+        var x = "AUTH8: "u8;
+
+        // UTF-16 encoded.
+        var y = "AUTH16: ";
+    }
 }