tests: add or delete copy/move ctors where needed to make type traits match reality (#5833)

tests/test_potentially_slicing_weak_ptr.cpp

@@ -11,7 +11,9 @@ namespace potentially_slicing_weak_ptr {
 
 template <int> // Using int as a trick to easily generate multiple types.
 struct VirtBase {
+    VirtBase() = default;
     virtual ~VirtBase() = default;
+    VirtBase(const VirtBase &) = delete;
     virtual int get_code() { return 100; }
 };