Add STM tests of the Unix module

[jmid]

The Unix module could also benefit from a good round of stress testing, e.g., to help ensure that it works consistently across platforms.

I made early experiments in this direction in https://github.com/ocaml-multicore/multicoretests/tree/unix-tests
A rebased version is available in https://github.com/ocaml-multicore/multicoretests/tree/unix-tests-rebased
These quickly found the analogous Windows rename issue plaging Sys.rename, confirming the usefulness of doing so.

Since parts of Unix strongly resemble Sys, to avoid duplicating code, this could benefit from the model-refactoring underlying the Sys test extension in #304, allowing the model to be shared among the two STM tests.