pybind11

Fix various minor memory leaks in the tests (found by Valgrind in #2746) (#2758)

* Fix leak in the test_copy_move::test_move_fallback

* Fix leaking PyMethodDef in test_class::test_implicit_conversion_life_support

* Plumb leak in test_buffer, occuring when a mutable buffer is requested for a read-only object, and enable test_buffer.py

* Fix weird return_value_policy::reference in test_stl_binders, and enable those tests

* Cleanup nodelete holder objects in test_smart_ptr, and enable those tests