Put matrix gen functions for inverse testing in a separate file to avoid duplication

[asobczyk]

These functions are repeated everywhere, we should put them into a single file. But we should do it in a separate PR.

Originally posted by @gioelegott in #107 (comment)

[zouzias]

I would suggest to put them under pto_kernels.testing, i.e., create a new testing Python module.

Then, we can import them inside the tests. We can even use a _testing name to make sure that users will not depend on it.