Adding a default virtual destructor to Animal type in test_tagbased_polymorphic.cpp.

With this change, and cast.h as-is in master, test_tagbased_polymorphic.cpp fails to compile with the error message below.
With the cast.h change in pull/2016, building and testing succeeds.

cd pybind11/build/tests && /usr/bin/c++  -DPYBIND11_TEST_BOOST -DPYBIND11_TEST_EIGEN -Dpybind11_tests_EXPORTS -Ipybind11/include -I/usr/include/python3.7m -isystem /usr/include/eigen3  -Os -DNDEBUG -fPIC -fvisibility=hidden   -std=c++2a -flto -fno-fat-lto-objects -Wall -Wextra -Wconversion -Wcast-qual -Wdeprecated -o CMakeFiles/pybind11_tests.dir/test_tagbased_polymorphic.cpp.o -c pybind11/tests/test_tagbased_polymorphic.cpp
In file included from pybind11/include/pybind11/attr.h:13,
                 from pybind11/include/pybind11/pybind11.h:44,
                 from pybind11/tests/pybind11_tests.h:2,
                 from pybind11/tests/test_tagbased_polymorphic.cpp:10:
pybind11/include/pybind11/cast.h: In instantiation of ‘static ...

tests/test_tagbased_polymorphic.cpp

@@ -12,6 +12,12 @@
 
 struct Animal
 {
+    // Make this type also a "standard" polymorphic type, to confirm that
+    // specializing polymorphic_type_hook using enable_if_t still works
+    // (https://github.com/pybind/pybind11/pull/2016/).
+    virtual ~Animal() = default;
+
+    // Enum for tag-based polymorphism.
     enum class Kind {
         Unknown = 0,
         Dog = 100, Labrador, Chihuahua, LastDog = 199,