C#: Add ref struct boxing example (false positive).

csharp/ql/test/library-tests/conversion/boxing/Boxing.cs

@@ -45,3 +45,6 @@ class C<T1, T2, T3, T4, T5, T6>
         x1 = x15; // not a boxing conversion
     }
 }
+
+// Ref structs can't be converted to a dynamic, object or valuetype.
+ref struct S { }

csharp/ql/test/library-tests/conversion/boxing/Boxing.expected

@@ -117,6 +117,9 @@
 | Nullable<Int32> | Object |
 | Nullable<Int32> | ValueType |
 | Nullable<Int32> | dynamic |
+| S | Object |
+| S | ValueType |
+| S | dynamic |
 | T1 | Object |
 | T1 | dynamic |
 | T3 | Object |