test: delete unused functions (code coverage purposes)

test/entt/core/any.cpp

@@ -19,7 +19,7 @@ struct empty {
     empty() = default;
 
     empty(const empty &) = default;
-    empty &operator=(const empty &) = default;
+    empty &operator=(const empty &) = delete;
 
     ~empty() {
         ++counter;

test/entt/entity/registry.cpp

@@ -66,8 +66,8 @@ struct destruction_order {
         *ctx_check = (registry->ctx().find<ctx_check_type>() != nullptr);
     }
 
-    destruction_order(const destruction_order &) = default;
-    destruction_order &operator=(const destruction_order &) = default;
+    destruction_order(const destruction_order &) = delete;
+    destruction_order &operator=(const destruction_order &) = delete;
 
     ~destruction_order() {
         *ctx_check = *ctx_check && (registry->ctx().find<ctx_check_type>() != nullptr);

test/entt/entity/storage.cpp

@@ -24,8 +24,8 @@ struct update_from_destructor {
         : storage{&ref},
           target{other} {}
 
-    update_from_destructor(const update_from_destructor &) = default;
-    update_from_destructor &operator=(const update_from_destructor &) = default;
+    update_from_destructor(const update_from_destructor &) = delete;
+    update_from_destructor &operator=(const update_from_destructor &) = delete;
 
     update_from_destructor(update_from_destructor &&other) noexcept
         : storage{std::exchange(other.storage, nullptr)},