C++: Regression tests for AV Rule 82

Without the last commit, this addition to the test gives the following
results:

```
+| AV Rule 82.cpp:176:14:176:22 | operator= | Assignment operator in class Forgivable does not return a reference to *this. |
+| AV Rule 82.cpp:181:14:181:22 | operator= | Assignment operator in class Forgivable does not return a reference to *this. |
```

cpp/ql/test/query-tests/jsf/4.10 Classes/AV Rule 82/AV Rule 82.cpp

@@ -171,6 +171,16 @@ class Reachability {
   static Reachability staticInstance;
 };
 
+class Forgivable {
+  // GOOD: wrong return type but that doesn't matter on a deleted function.
+  Forgivable operator=(const Forgivable &_val) = delete;
+
+private:
+  // GOOD: wrong return type but that doesn't matter because this operator is
+  // private and probably also unimplemented, so no code can call it.
+  Forgivable operator=(int *_val);
+};
+
 int main() {
   Container c;
   c = c;