Run tests on all xtensor objects

[tdegeus]

It might be worth to have set of tests for which we use the preprocessor to run them on xarray, xtensor, xtensor_fixed, pyarray, pytensor, ... This should reveal minor issues such as solved in https://github.com/xtensor-stack/xtensor/pull/2510

The only thing that I'm not entirely sure of is what is a nice practical way to approach this as some of the objects run on a different repo. In particular it would be nice to cast minor issues before releasing, i.e. it would be nice to check based on each of the masters, and run when either of them is updated.

[JohanMabille]

A solution could be to create a new repo forgeneric tests that can accepts array / tensors from the core library and the language bindings. Then all the repos would depend on it for common tests and would implement their specific tests only.

[tdegeus]

That is indeed what I would do too. I was just worried that we would be relatively lazy in updating it, being an external library.