Complete tests for "17.21.3 Superinvocations"

We have no tests covering the case when 

```dart
abstract class A { 
  void m(); 
}

abstract class B extends A { 
  void n() { 
    super.m();  // The method 'm' is always abstract in the supertype. • abstract_super_member_reference
  }
}
```
Possibly there are other missing cases as well