Fix tests built with PYBIND11_ASSERT_GIL_HELD_INCREF_DECREF.

There was more (hidden) calls to inc_ref/dec_ref while the GIL was not held.

All of them are lambdas used as callbacks with a captured py::object.
As a workaround, the reference counter is increased before releasing the GIL,
and the lambda simply captures the raw pointers.

For more details, see the release notes for pybind11 versions 2.10.2, 2.10.3,
and the future 2.11.