test: fix warning in test

Author: iboB

test/t-ref_ptr-11.cpp

@@ -72,6 +72,7 @@ TEST_CASE("ref_ptr") {
 
 struct animal {
     animal(std::string n) : name(std::move(n)) {}
+    virtual ~animal() = default;
     std::string name;
     virtual std::string speak() const = 0;
 };
@@ -105,4 +106,4 @@ TEST_CASE("unsafe") {
     auto sp2 = rp._as_shared_ptr_unsafe();
     CHECK(sp == sp2);
     CHECK(rp.use_count() == 3);
-}
+}