test_eigen.py test_nonunit_stride_to_python bug fix (ASAN failure) (#4217)

* Disable test triggering ASAN failure (to pin-point where the problem is).

* Fix unsafe "block" implementation in test_eigen.cpp

* Undo changes (i.e. revert back to master).

* Detect "type_caster for Eigen::Ref made a copy."

This is achieved without
* reaching into internals,
* making test_eigen.cpp depend on pybind11/numpy.h.

* Add comment pointing to PR, for easy reference.