Make content of test_common.hpp reusable for third-party projects

[SylvainCorlay]

At the moment, xtensor, xtensor-python, xtensor-julia and xtensor-r all have their flavor of this file containing testing utilities for container expressions.

We should probably make it installable (maybe as a separate target).

Also, we have made some changes in the third-party projects fixing some warnings due to integer types (xtensor-python and xtensor-r have signed size types), which would be ported to this unique implementation.

cc @JohanMabille