Adding test_class_sh_factory_constructors.cpp to tests/CMakeLists.txt (fixes oversight, this should have been added long before).

Adding construct() overloads for constructing smart_holder from alias unique_ptr, shared_ptr returns.

Reverting commit 63495313 by un-commenting `const` in `def_buffer(...)`. To make this possible, `operator T const&` and `operator T const*` in `smart_holder_type_caster` need to be marked as `const` member functions.

Removing unused smart_holder_type_caster_load::loaded_as_rvalue_ref (it was an oversight that it was not removed with commit 23036a45).

Systematically changing std::enable_if back to detail::enable_if_t, effectively reverting commit 5d4b6890.

DEBUGGING_MSVC_2015: simplification (from compiler perspective) of struct is_smart_holder_type_caster implementation.

Uncommenting `#define PYBIND11_USE_SMART_HOLDER_AS_DEFAULT` to run GitHub CI with smart_holder as default. NOT meant for merging.

Systematically replacing `detail::enable_if_t<...smart_holder...>` with `typename std::enable_if<...smart_holder...>::type`. Attempt to work around MSVC 2015 issues, to be tested via GitHub CI. The idea for this change originates from this comment: https://github.com/pybind/pybind11/issues/1616#issuecomment-444536813

Also removing `operator T()` from smart_holder_type_caster, to fix gcc compilation errors. The only loss is pass_rref in test_class_sh_basic.

Using @pytest.mark.parametrize to run each assert separately (to see all errors, not just the first).

Fixing unfortunate editing mishap. This reverts the last remaining test manipulation in commit 249df7cb and makes all existing unit tests pass with smart_holder as default holder.